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Old 26th Jan 2018, 11:00 am   #1
Diabolical Artificer
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Default 2C34 amp.

Whilst saving up for an OPT for the big amp I built, I was after another project. I rumaged amongst my valves and tfmr's and settled on trying to use the 2C34 triode. After reading Tricomp's thread - https://www.vintage-radio.net/forum/...ad.php?t=67511 , I thought I'd give it a bash.

His amp is in class AB2 and he uses some non standard valves on the front end and PS/driver, I only have your bog standard valves namely the ECC triodes and octal equivalents as well as a few pentodes. I have quite a few 6H8C Russian 6SN7's, so thought I'd use them. The only problem there might be is that we need to drive the 2C34's into positive grid current, this really needs a beefy valve like the 12BH7 but because I like to make life harder for no other reason than trying to use only octal valves, I'm stuck with the 6H8C, which should be ok, maybe. There are hefty octal triodes, but I don't have any at present.

Anyhoo, I knocked up a simple circuit using one half of a 6H8C as voltage amp with a 150k anode resistor, 2k2 cathode R, no bypass cap, followed by a direct coupled cathodyne using 22k R's top and bottom. With a HT of 300v for both, I get an OP of 60v P-P for a IP of 1v RMS ish, so not brilliant, but should be just enough as I need 100v g1 - g1 on the 2C34's.

The OP of the cathodyne is de-coupled to two cathode followers, 20k cathode resistors, biased at around -7v, so around the midpoint. The OP is then de-coupled to the OP stage; the 2C34 has two 47K grid resistors connected to a -15v DC fixed bias/bench PSU.

I'm getting a 15v P-P OP, around 3.5w, so could do better, but I threw this circuit together mainly to test the OPT's I have.

Here lies part of the problem I think. I have two OPT's. the one I'm using at present is a Wharfdale with a secondary that can wired to get 15/22/45:1 Z ratio. I'm using the 22:1, which gives a Zpri of 3K8 for 8ohm load. The 2C34 datasheet specs a Zpri of 10k. I tried the 45:1 tap (16k ish Zpri), this being a bit too much, OP was reduced.

Tricomp uses a direct coupled CF to his OP stage with constant current sinks fed by a negative voltage -82v. I've experimented with this, but found I have to bias the valves nearly into cutoff to get the cathodes to sit at -15v.

He also uses an odd PS which I can't quite get my head round, it looks like a floating cathodyne or maybe a paraphase. I've never used the cathodyne PS before, as it has little gain, prefering to use an LTP. The problem with the LTP is that it needs an extra triode, so you end up with using only one triode of a double triode valve as the input/gain stage - wastefull.

I'm off to try the other OPT I have which has two separate secondary's and a higher Z ratio, but the circuit as is can drive the 2C34's but needs a pentode on the front end I think, maybe a EF37 triode strapped to give more headroom. I'm just winging it present, prefering to get a circuit working then refine it, rather than work out everything in advance.

Any comments/ideas welcome as always, Andy.
Attached Files
File Type: pdf 2C34-PP_5687WA-CF_CCS_Final.pdf (15.8 KB, 205 views)
File Type: pdf 6AN8A_Pre_Inv_Final.pdf (12.4 KB, 132 views)
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Old 26th Jan 2018, 2:29 pm   #2
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Default Re: 2C34 amp.

Made a few more tests, but first I checked the cathode current on the 2C34 which was 38mA at rest, but dropped to 28mA at full OP. Suspecting the mains tfmr/PSU I used my big bench PSU capable of 500v 3A.

Checked quiescent I again, it was 43mA but dropped to 34mA at full OP which was 4.6v RMs which is a paltry 2.6w. This is with a Z pri of 16.2k - 8 ohm load. Swapped sec taps to get a Z pri of 8k with a 16 ohm load, OP dropped to 3.1v RMS.

Voltage on the grids of the 2C34 is 19.5v RMS or g - g of 39v RMS or 109.98v AC P-P , this should be more than enough to drive the OP valve into positive grid current/AB2; AFAIK, it is. The load line for either Z pri suggests we need a swing of up to -24v to +24v P-P which were getting. Can't work out why cathode current drops at full OP, it should go up surely.

Lastly changing the bias of the OP valve changes nothing much. The OP is not clipping but develops crossover distortion. Heads swimming a bit, so am probably missing something. Need to look at my old notes for the big amp and have a think, but I suspect the OPT isn't optimum for the job. I'll draw up a schematic later.

TFL, Andy.
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Old 27th Jan 2018, 12:19 pm   #3
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Default Re: 2C34 amp.

The January Mr Dopey award goes to AW Scredington : ). Firstly Ik should be halved as the 2C34 has a common cathode and 12 ohm sense R, so Ik is 16.5mA. Secondly I boo boo'd with the grid R's on the 2C34, I had two 47k R's, one on each grid, the ends connected to -15v, so no reference to ground. I've put them as per the schematic.

A mistake I saw whilst drawing the schematic is the OP from the CF should be from the 10k/1k5/470k junction, not the cathodes themselves. The circuit was thrown together so caps and R's arn't optimum for frequency response. Room for improvement.

Followed Jeffrey's suggestion, over on Goldbourne, scoped one anode of the 2C34. There's a load of high frequency crap on there as well as 50hz. It looks like a 33khz signal modulated by 50hz. 50hz from the heaters I guess, but layout isn't optimum.. Will try bigger stoppers and ferrites.

The main problem seems to be crossover distortion. I monitored the common cathode of the 2C34. It's 33mA (both) at rest, rises to 37mA then drops to 30mA at full OP. Xover dist occurs soon after Ik starts to drop. Wondering if there's some blocking distortion going on. Also CF could be better, will redesign.

See oscillograms below. Top trace OP , bottom one anode of 2C34.

Andy.
Attached Thumbnails
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Old 27th Jan 2018, 3:26 pm   #4
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Default Re: 2C34 amp.

I was going to make a quip that going to a 2C39 might give faster bass, but I found someone had already been there.

It looks lethal!

https://www.youtube.com/watch?v=mWKCUd6BUVk

With the RK34, you may be better having two running push-pull parallel pairs to separate the cathodes and let you have a bit more freedom in the bias department. I did this back in the sixties in an all-RK34 amp because I had boxes of 'em.

David
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Old 27th Jan 2018, 4:15 pm   #5
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Default Re: 2C34 amp.

Given what the 2C34 was designed for - a VHF push-pull oscillator/amplifier - I can easily understand how you're seeing 'unexpected' oscillations.

Your amplifier is merely achieving its underlying ambition to become an oscillator while your back's turned.

What are you using for the 'stopper' resistors? The classic way to tame VHF parasitics in amplifiers was either to use low-value [50-100 Ohm] high-power *wirewound* resistors, or to take a reasonably-powerfully-rated [5-watt?] Carbon resistor of 50-100 ohms and wind along it a number of turns of something like 25-gauge enamelled wire, soldering the ends of this wire to the two resistor ends.

Either way made a "RF-lossy choke" which - if fitted as close as possible to the grid- and anode-connections worked as an effective stopper. Just don't use identically-proportioned stoppers in both the grid and anode circuits or you could be producing a classic 'tuned-anode-tuned-grid' oscillator.

Ferrite beads slipped over the grid- and anode-leads often helped too.
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Old 27th Jan 2018, 7:28 pm   #6
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Default Re: 2C34 amp.

It is perfectly normal to start to feel that you've added more stopper components than the circuit has normal operational components.

When you use a rather frolicsome device at audio, you still have to give it nice, calming impedances across the RF spectrum.

The quality of construction is not set entirely by the frequency you intend to use it at, it has to be right for the entire range that device could get into trouble at.

David
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Old 28th Jan 2018, 8:09 am   #7
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Default Re: 2C34 amp.

"With the RK34, you may be better having two running push-pull parallel pairs to separate the cathodes"
Could you elucidate please David?

I have some RF chokes somewhere which I've been threatening naughty valves with on occasion, but I want to get the DC conditions sorted and do some work on the front end and drivers, tidy the layout etc (see pic).

I redesigned it yesterday on paper, did everything properly - it doesn't work or should I say it's ble*ding awful. Typical, throw it together using anyoldbits and it works.

BTW, how does a RF/VHF valve differ from one designed for AF in construction?

Later, A.
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Old 28th Jan 2018, 9:19 am   #8
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Default Re: 2C34 amp.

An output stage with four triodes. Each parallel pair being from one bottle. Each side of the push-pull having a cathode bias resistor, either decoupled to ground or decoupled across to the other side.

RF chokes aren't as broadband as lively RF devices. Often their resonances make things worse. With spurious oscillations, it's resistors which are usually needed. Adding L or C moves a resonance around and adds extra modes. Adding R opposes the negative resistance which powers the oscillation.

David
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Old 29th Jan 2018, 2:55 am   #9
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Default Re: 2C34 amp.

I agree with Radio Wrangler's remarks about resistors to knock out spurious oscillations, not inductors. Although in many audio circuits grid stoppers of 100R are used, they can be as high as 47k for audio work without compromising the upper audio frequencies depending on the valve's input capacitance.

Interesting to see the SC502 scope posted by Andy in Post #3. I have one too. This scope has one of the more interesting and excellent internal construction/design builds of any scope. To look in one is a humbling experience to see how TEk managed to put in all the required electronics in a small enclosure in such an orderly and well thought out manner. It is analogous to a well written symphony.

When I think I have built some machine properly and my head starts to swell, I look inside this scope again to bring me back down to Earth and realize that anything I could design & build is not even half as good as what Tek achieved with this little beauty of a baby scope.
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Old 30th Jan 2018, 7:40 am   #10
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Default Re: 2C34 amp.

Yes the SC502 is the pinnacle of design. I'm fortunate enough to have the manual; i sit in bed reading it like a novel.

After some alteration's resulting in going backwards I got progress of sorts on the amp and got 8v RMS out of it (8 virtual watts) yesterday at 0.7% THD. However the OPT I have isn't optimal, I had to bung a 23 ohm load too bump the Z pri up to 11k to squeeze that out.

Also bunged a speaker on the other end, it sounds ok but didn't result in a silly grin. However there's no hum or hiss and scoping the anodes of the 2C34 and OP shows no nasties. I had to put the 10x magnifier on (0.001v/div) to find anything. This showed some spikes and 50hz but no HF of VHF osc.

My big PSU has some nasty spikes on it, but it is a triac controlled PSU. It makes some odd noises too. It bangs and clicks.

I think there's an impedance issue between CF's and OP valve, will have another go at it today, have a few ideas.

A.
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Old 31st Jan 2018, 8:21 am   #11
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Default Re: 2C34 amp.

Hi Andy and all,

I think everybody missed the point of class AB2. The "2" means grid-current and doing that you can't have any grid-stoppers as they would drop the grid-current that should be as stiff as possible, being driven from as small an impedance as possible. That's the point of using DC-COUPLED Cathode-Followers to drive the 2C34's. DC-Coupled as in my schematic, Andy posted in the opening post. NO COUPLING Cap's between the CF stage and the 2C34 grid! When doing that RF-oscillation and instability goes away instantly. Take another look at my schematic. The whole bias-setting circuitry should be moved from the 2C34 input to the CF grids. Between the CF and the 2C34 grid is ZERO ohms and NO cap's or AB2 will not work.
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Old 31st Jan 2018, 8:46 am   #12
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Default Re: 2C34 amp.

That's the nasty compromise. You need grid stoppers to quench the Q of spurious resonances, but they can't be too big or else you drop volts in them when drawing grid current, and the drive t the device is less stiff.

Anode stoppers rob you of output power.

There is no easy victory. RF oscillations rob you of power, get self-rectified, cause DC offsets and distortion.

Getting an amplifier stable with high-frequency-capable devices is an art.

It gets even harder for RF amplifiers when there is less space between the wanted frequencies and the spurii.

David
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Old 31st Jan 2018, 9:11 am   #13
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Default Re: 2C34 amp.

Quote:
Originally Posted by Radio Wrangler View Post

There is no easy victory. RF oscillations rob you of power, get self-rectified, cause DC offsets and distortion.

Getting an amplifier stable with high-frequency-capable devices is an art.
In Audio-circuits there is an easy victory: CF DC-drive !

I battled with a push-pull output-stage using a couple of 6AD10 compactrons.
The stage was all over the place when driven the usual way with high impedance input, capacitor-coupled from a cathodyne phase-inverter. I tried everything in my testbed, even using tinfoil as shielding but to no avail.
Then I decided to try my 2C34 CF-drive from pervious and the output stage went rock solid. Absolutely NO sign of instability no matter how sloppy wiring was done in the test-bed.
You don't HAVE to drive the output valves into grid-current when using the CF-drive but the option is there and it sounds great if you decide to turn up the volume beyond reasonable. I believe I was happily running the 6AD10's at 28Watt anode dissipation and it's built for 12Watt. Beyond 28Watt they red-plated, so I backed down a bit.
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Old 1st Feb 2018, 1:08 am   #14
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Default Re: 2C34 amp.

The problem with cathode followers is that its is hard to get the output impedance below a few hundred ohms, so the output from these, compared to semiconductor followers, always have a higher Z, so if you added 47R grid stopper resistors, it would hardly make any difference to the overall source impedance of the driver, and there will still be a voltage drop due to grid current, and at least you would get some RF suppression.So there might be a compromise in there. In general the way this was handled in vintage class AB2 audio amps was a step down (rather than a step up) transformer to drive the grids.
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Old 1st Feb 2018, 7:53 am   #15
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Default Re: 2C34 amp.

Transformers maybe in very vintage amplifiers but there are lots of less vintage amplifiers doing it the CF way.
The Heathkit W-6M springs to mind for starters, see the attached.
Even as far back as 1947 Audio Engineering showed how to use a CF-drive on 2A3's or 6B4G's in push-pull with sliding bias from class A to class AB2, also attached.
Present day Bespoke Audio Co. also seems to use CF-drive on 807's, also attached. However, I can't find any ref. to that amplifier on their web-site, so may not be genuine.

Common to all is no grid-stopper as imho it's not needed.
Driving the grid from a low-Z source tames the beast.
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Old 1st Feb 2018, 8:17 am   #16
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Default Re: 2C34 amp.

Thanks for the comments lads. I'm still having trouble with this amp, which I'll get to in a sec. I looked at
using direct coupling the CF's to the OP stage as Tricomp did, but am bu**erd if I can see how it works because to get the cathodes to sit at -15v any valve be it a 5687, 12BH7, ECC99 needs biasing right near cutoff to get the cathode so low.

The idea of a CF to drive any thirsty OP stage and especially here is to bias it at the midpoint or higher to get high current - 10mA or higher as the 2C34 needs 500mW to drive the grids.

After making a few changes to the layout, putting ferrites on the grids and anodes and putting bigger grid stoppers - 2k2 and using a 12BH7 as the CF's, set to -9v bias 10mA (more or less the same as on the big amp i built, that drove 3 EL34's. so should be sufficiant to drive the 2C34) I tried again.

Still the same trouble. Whilst turning up the volume I monitored cathode current of the 2C34 through a 10 ohm R. At rest we get 32mA, as it's a shared cathode that's 16mA per triode, which agrees with my load line (it often doesn't). As I turn up the wick Ik rises to around 42mA, then drops. The OP sine still grows bigger in amplitude, but crossover distortion occurs soon after. I also noticed at a tad over 1khz, there is a big notch bitten out of the left hand side peak.

To see if it it's oscillating I tried using the notch filter of my distortion meter to filter out the AF then scoped the OP of the meter, I could find no significant oscillation up to 80Mhz, but did find 60khz at 1mV P-P, surely not the culprit. So wondering if my meter's circuits could handle that higher bandwidth or the osc was higher (the 2C34 can go up to 240Mhz and higher) I hooked the OP up to a frequency counter which goes up to 2gig - nothing.

So I'm leaning towards a problem with the CF stage, Tricomp's findings back this I think. I'm still wary of the direct coupled approach, but thought I'd give it a go. This means using a CCS, but I'm struggling to find the OP Z of the CF with CCS. I'm using a MJ340, but to calculate Zout you need hoe and hfe, both are not listed on the datasheets I've looked at for the MJ340. I've looked to see if I can calculate these parameters, but the search engine has let me down.

So meanwhile I completely took everything apart and built a new test jig as the old ones had wires all over the shop and unscreened cable. Will report back.

Later, A.

Posted same time as T. Thanks mate, will have a shufty at those.
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Old 1st Feb 2018, 10:27 am   #17
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Default Re: 2C34 amp.

Quote:
Originally Posted by Diabolical Artificer View Post
I looked at using direct coupling the CF's to the OP stage as Tricomp did, but am bu**erd if I can see how it works because to get the cathodes to sit at -15v any valve be it a 5687, 12BH7, ECC99 needs biasing right near cutoff to get the cathode so low.
Just mount a 22~47KOhm resistor as a replacement to the CCS between CCS IN and OUT (Pins 2 and 3 in my schematic).
You can worry about the CCS later, it's just an improvement, not an absolute necessity.
To get things going with the DC-coupled CF, it's a starter.
Then wire up the biasing circuit to the CF input-grid.
You may have to twiddle the value of the bottom 100KOhm common resistor to ground in my schematic, to get a sane setting on the bias-pots.
Sane = mid-settings when the 2C34 grid is biased at -15VDC from the CF.
Then you can bias up and down as required.
It's really quite simple once you get into range and that's where the bottom 100KOhm resistor and the negative bias-voltage level all has to come together to provide the -15V for the 2C34's.

About the 5687 valves I think any reasonably substantial double triode may do as a replacement. This includes 12BH7, ECC82/12AU7 and octal equivalent 6SN7GT.

EDIT:
When fiddling to get the CF output voltage within range, i.e. supplying -15V to the 2C34 grids, just leave the output valves out. Pull from sockets, simply. Then you don't have to worry about what current they may draw when incorrectly biased. Once you get -15V on the CF cathodes, plug the 2C34's back in and all's well.

Last edited by tri-comp; 1st Feb 2018 at 10:34 am.
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Old 2nd Feb 2018, 2:31 am   #18
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Default Re: 2C34 amp.

Quote:
Originally Posted by tri-comp View Post
Transformers maybe in very vintage amplifiers but there are lots of less vintage amplifiers doing it the CF way.
The Heathkit W-6M springs to mind for starters, see the attached.
The Heathkit combines the CF with an ultralinear output transformer connection too. Probably quite a good amp that one.
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Old 2nd Feb 2018, 8:27 am   #19
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Default Re: 2C34 amp.

Spent yesterday sorting the layout.. again and knocking up another CCS. It's tricky to bias a CF with a CCS, I'm using a potential divider with the grid R's as the bottom leg's. One mistake I found was my neg bias supply. I'm using a separate tfmr and had a potential divider to bring the voltage down. With a 10k fixed R and 5k pot this meant the CF's cathode's were starved of current. Don't think this is the main problem though.

I was reading part 2 of Simon Westinghouses article on valve amps yesterday morning which has some relevance on my problem. In a nutshell he sais positive grid current charges coupling caps increasing neg bias due to current flow from grid to cathode. He puts it better and in more detail here - http://sound.whsites.net/valves/analysis.html ....part 9.

Onwards and upwards, A.
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Old 2nd Feb 2018, 10:49 am   #20
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Default Re: 2C34 amp.

Quote:
Originally Posted by Diabolical Artificer View Post
In a nutshell he sais positive grid current charges coupling caps increasing neg bias due to current flow from grid to cathode.
Yes if there is grid current, you require DC coupling, from a CF, or you need a transformer drive. Any capacitive (AC) coupling will foul up the bias as the grid current is unidirectional and will charge the capacitor shifting the average bias depending on signal level.

While there can be benefits in power output with class AB2, in all of my valve audio amplifier designs, I have always avoided grid current in all stages, as I think this type of amplifier is more suited to industrial and power servo applications than audio. (hope I don't start a fight with that remark, just my opinion).
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