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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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2nd Feb 2018, 2:03 pm | #21 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
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Re: 2C34 amp.
To avoid using AB2 don't you need valves capable of passing enough anode current at zero grid volts to give you around double their PA dissipation rating? Otherwise the best power out you can get will be running the valves quite conservatively.
I guess it's a problem with using valves not intended for audio PA duty but with temptingly high PA ratings.
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2nd Feb 2018, 10:07 pm | #22 |
Heptode
Join Date: Oct 2008
Location: Aalborg, Denmark
Posts: 903
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Re: 2C34 amp.
Actually 2C34 IS specified for audio, not only for RF.
I agree it's not specified for Hi-Fi, but then again that term wasn't invented at the time the valve was introduced afaik. CF-drive also not invented I believe. Datasheet attached (RK34 and 2C34 are 100% alike) Btw, the 2C34 is electrically very similar to the 6A6 and 6N7 dual power-triodes. It's a dual power-triode with modest filament requirements, it looks good, it's by FAR less expensive than doing a similar push-pull with 2A3 or 300B AND it works (for me ) What's there not to like ? |
3rd Feb 2018, 8:39 am | #23 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,667
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Re: 2C34 amp.
Sussing out what happens when an OP valve goes into grid current is also relavent when driving "normal" AF valves in AB1 Chris, as they go into or near grid current before clipping. See Keibert - System Design Facrors for AF Amplifiers below. Bit hard to follow in places but.
Well I had some success yesterday, with about 9v RMS OP, no fall in Ik. This is after spending a day or so trying to get a CF with CCS working. I tried to bias the CF using a voltage divider, which is nigh impossible as of coarse the cathode follows the grid, in this case by about 13v. It then occured to me trying to draw a loadline as per usual is a waste of time. I'm sure the grid lines are nearly horizontal too and have a feeling that if this is the case, then the load line will be near vertical rather than slanting ; a bit like a class A PP SE stage. So I used fixed bias to bias the CF's and since I was doing this I though I may as well remove the coupling caps and direct couple the CF's as per Tricomp's design. I've been reading more on this approach from vintage sources and think it's the only way to do it. Got the cathode's sitting at -15v, signal in, scoped the OP of the CF's.... horrible! Looks like someone had punched the bottom peak on the nose and the peak was shoved inside, rectification? Also low amplitude. Gave it some more current on the CCS, sine slowly "re-forming", amplitude up. to get the OP of the CF right I found I had to set the CCS to 180mA! Not sure this is right but the MJE340 is within SOA as ce is around 60v, so something like 11w. Still not 100% sure what the craic is with the CCS "driven" (should that be sourced, more like sunked) CF, but will investigate further today if I get time. When I get it working I'm going to put a coupling cap back on and grid stopper, I want to get to the bottom of this AB2 lark. Andy
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3rd Feb 2018, 8:59 am | #24 |
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Re: 2C34 amp.
Yes that is correct. I like to run my valves very conservatively, it extends their life. Also, on the whole, the amplifier stages have lower distortion and less overall negative feedback is required. This is made easier with modern & efficient speakers.
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3rd Feb 2018, 9:09 am | #25 |
Dekatron
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Re: 2C34 amp.
But using normal AF valves is less of a challange, the 2C34 needs to be run over 0v grid to get usable power out.
A.
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3rd Feb 2018, 9:57 am | #26 | |
Heptode
Join Date: Oct 2008
Location: Aalborg, Denmark
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Re: 2C34 amp.
Quote:
Also, 180mA through the CCS is impossible. That current should go somewhere and it's NOT disappearing into the 2C34 grid or for that matter into the CF drive valve (...unless you're using an EL519 ) More likely you're running 18mA. |
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3rd Feb 2018, 10:02 am | #27 | |
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Re: 2C34 amp.
Quote:
To give an example, starting back in the mid 1920's, a valve like the UX171 was designed for class A amplifier use. To get it to run successfully in class AB1 push pull, it requires around -45 to -55V grid volts, to avoid the grid going + with respect to the filament (cathode) when full output power is realized. Later, a tube like the RCA type 49, designed for class B use, required zero grid volts and was intended to draw grid current on nearly the whole part of each half cycle of its grid drive voltage. The design of some valves is that they are not as suited to Hi Fidelity audio amplifier use as others, if one is wanting to extract power from them in an output stage. Of course you can press them into use, valves that require grid current for any reasonable power output, but along with that comes more distortion and the requirement for more overall negative feedback to reduce it and power transfer from a "power driver stage". I've noticed over the years as audio suited valves have become more expensive, there has been a tendency for people to press other types of valves, for example RF types (often intended for class C applications) into audio service. So clearly these work better in class AB2 than an audio valve designed for class A. Also, when it comes to grid current, the grid impedance of valves as the grid goes positive with respect to the cathode, is quite different depending on the architecture of the specific valve design and the effects of it are more abrupt in some than others. Still, not nearly as abrupt as the gate to source impedance of a J-Fet, when you take the gate more positive than the source. In any case, the drive stage (whatever it is, transformer or CF or anode circuit) always has a finite source impedance and grid current loading it is always "less than ideal". Still, the output valves in many commercial class AB1 amps do draw grid currents on peaks at full output, but it is less than ideal, if amplifier fidelity (absence of amplitude & phase distortion) is the aim of the game. Last edited by Argus25; 3rd Feb 2018 at 10:10 am. Reason: typo |
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4th Feb 2018, 8:33 am | #28 |
Dekatron
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Location: Sleaford, Lincs. UK.
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Re: 2C34 amp.
Yes, the 2C34, poor mans OP triode, no way I can afford a 300B. "grid current loading it is always "less than ideal". " Why?
So yesterday after more mucking about I got 13v RMS OP, but have noticed distortion around 1500hz which goes away as frequency increases. F response is pretty flat from 15hz to 17khz with about 1% THD, no NFB. I calculated the CCS current by what I had as the emitter resistance after adustment, IE 30 ohms. I have a 300 ohm preset. Did have a 220r and the preset, but shorted the fixed R out after I ran out of adjustment. With a 6v2 zener as reference that's 5.6 divided by 30 = 189mA. Obviously this is suspect so I put a 100 ohm R between the collector and cathode and measured 0.149v = 1.4mA. this too seems wrong. Bog knows, it's working anyway. A.
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4th Feb 2018, 8:54 am | #29 | |
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Re: 2C34 amp.
Quote:
But I could add it is not so bad if you get grid current for the complete half cycle, it is when it transitions from no grid current to grid current that most of the non linearity occurs. Also, a valve running in a grid current mode, can no longer be thought of as a transconductance device. Last edited by Argus25; 4th Feb 2018 at 9:17 am. |
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4th Feb 2018, 4:45 pm | #30 | |
Heptode
Join Date: Oct 2008
Location: Aalborg, Denmark
Posts: 903
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Re: 2C34 amp.
Quote:
I'm confused about CCS resistor values you mention. What 220R emitter resistor are you referring to ? In my schematic it's 2KOhm. Measuring the voltage drop across it and you may calculate what current it carries in the normal way. That current will be the CCS current and the standing current of the CF. I built and calculated the CCS following schematic "d" in the attached Morgan Jones excerpt. |
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4th Feb 2018, 6:29 pm | #31 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
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Re: 2C34 amp.
How about looking at running it with the grids always-positive?
It was done extensively in the 1930s, indeed there were valves specifically designed for this kind of service. http://www.r-type.org/exhib/abi0041.htm https://www.radiomuseum.org/tubes/tube_6b5.html |
4th Feb 2018, 8:32 pm | #32 |
Heptode
Join Date: Oct 2008
Location: Aalborg, Denmark
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Re: 2C34 amp.
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4th Feb 2018, 10:33 pm | #33 |
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Re: 2C34 amp.
Raytheon quote Class-B audio, 13W output. 8k Ohms anode-to-anode, 70v RMS grid-to-grid and -15v grid bias. 300v DC HT
That's about 15mA per side quiescent so it's a bit AB inclined. But that grid drive 35v RMS on each side will peak with the grid 34.5v positive which wil give significant grid current. Like any data sheet, it states the drive at the device without explaining how difficult it is to achieve. david
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5th Feb 2018, 7:43 am | #34 |
Dekatron
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Location: Sleaford, Lincs. UK.
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Re: 2C34 amp.
Thanks Argus, that explains thing's.
My CCS is different to your's T, it only has one tranny, a MJE340. There is a 6v2 zener from base to neg volts, fed by a dropper resistor to supply base current and zener current, with an emitter R to set CCS I. To set CCS current is V zener - be volts, (so 0.6 ish) = 5.6v, then use ohms law to get emitter R. Those are about the figures I'm getting David but my HT is 320v. The output sinewave has distortion on the leading edge of the positive peak as well as rounding of the peaks prior to and at full whack. my distortion meter says 1% thd @ 1khz, which drops to 0.8% with a bit of NFB. Still only experimenting with a pot, so can't be precise. Squarewave test at 1k and 10k shows significant rounding to the tops as well as sloping. There is a chunk bitten off the top pos peak on both the anode sines, which to a point is getting cancelled out. I'll take some pics later. One last thing, I'm having to put in 2.1v RMS to get 12v RMS out, so front end needs looking at. Using a pentode as Tricomp did or an ECC83 would help, or as T also did use a triode/pentode for the in/ps. Later, Andy.
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5th Feb 2018, 10:52 am | #35 | |
Heptode
Join Date: Oct 2008
Location: Aalborg, Denmark
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Re: 2C34 amp.
Quote:
Read the Morgen J attachment from pervious post, if you didn't already. The output impedance of you CCS is too low to be any good. You may as well use a simple resistor instead. I expect my CCS has an output-Z past 1MOhm with conservative data for the MJE340. |
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5th Feb 2018, 11:22 am | #36 | |
Heptode
Join Date: Oct 2008
Location: Aalborg, Denmark
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Re: 2C34 amp.
Quote:
Look at the yellow curve. The top clearly shows a bend/kink in the waveform when grid-current enters. This was not taken from my 2C34 amplifier which hasn't been examined in details for this state, but from another amp. I did. I wouldn't be surprised if it shows something similar. In that case I would experiment with the standing current of the CF to minimize the kink. |
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5th Feb 2018, 2:19 pm | #37 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
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Re: 2C34 amp.
Got you re the CCS. I couldn't calculate Z as try as I might I can't find hoe and hfe for the 340. I'll re-read MJ's artical on the subject.
Here's some oscillograms. Applied more NFB today, which makes things a bit better, but as you can see the OP of the voltage gain triode isn't brilliant. A.
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5th Feb 2018, 3:44 pm | #38 |
Heptode
Join Date: Oct 2008
Location: Aalborg, Denmark
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Re: 2C34 amp.
Calculating the CCS Z-out with MJE340 and MPS-A06 as active elements.
MPS-A06, Hfe at Ic = 3mA --> ~150x MJE340, Hfe at Ic = 3mA ----> ~40x (All at 25C) CCS Z-Out = Re x Hfe(lower) x Hfe(upper) = 2000 x 150 x 40 = 12MOhm. Disregard 1/hoe according to Morgan Jones. Transistor datasheets attached. |
5th Feb 2018, 7:33 pm | #39 |
Heptode
Join Date: Oct 2008
Location: Aalborg, Denmark
Posts: 903
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Re: 2C34 amp.
Here's some Japanese CF-drive ingenuity: Fully DC-coupled Push-Pull amplifier.
Not the Loftin-White way but the Aikawa Hideo way (Whoever he is) Look at this 1987 patent featuring a lot of CCS' and the CF-drive. With a CCS in the output stage it must be running class A, so why the CF-drive... or, perhaps why not ? Does the plenty use of CCS' keep the circuit stable when valves age ? |
6th Feb 2018, 10:23 am | #40 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
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Re: 2C34 amp.
His idea seems to be direct couple everything so no phase shift etc and keep the PSU rock solid, but as he sais that presents problem's namely an increasing ground plane which needs a very high voltage at the OP stage.
"Does the plenty use of CCS' keep the circuit stable when valves age ?" I've read something to that effect. Thanks, I have that datasheet, as you see no small signal gain only hFE. So how does one compute hfe for another Ic? Spent hours looking through books and online but the hated browser kept giving me everything but what I wanted. The damned things just try to sell you stuff all the time. Maybe I'll try google scholar. Valves are a doddle compared to tranny's. Complicated little b*ggers. A.
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