Mini Gee-Tah amplifier

[tri-comp]

The amp's converted back to guitar-use.
I can't really decide on how to configure the pentode driving the phase-inverter.
Of course with self-bias and de-coupled cathode-resistor total voltage amplification in the amplifier is too big with the input triode in the circuit.
For now I solved that problem by not decoupling the pentode cathode resistor.
Amplification reduced and sound quality improved in that particular link.
Total amplification suits guitar pick-up level from 50mV to 100mV.

This evening I'll have a pro guitarist over for a beer. He doesn't know what awaits him (...except for the beer )
I'll pick his brains until there's none left
Of course I run the risk og him fleeing over the hills once he hears what emits from the amp....

Next report will have an updated schematic; contains much depending on the outcome of the above mentioned session.

TFN

/tri-comp

[tri-comp]

OK, beer's gone and so is the guitarist.
NOT fleeing over the hills...
First I invited him w/o his own guitar as I have a Vester Tradition copy of a Fender Stratocaster.
When he played it THAT almost send him running.
Well, actually he did go home to pick-up his own Burns Marvin re-issue while I prepaired coffee; beer long gone.
I clearly see what he means when he started playing the Burns. It sounds VERY different as in WAY better!
We did some adjustments to the amplifier and used NO other helping with fuzzers, reverbs and the likes. Just the naked guitar fed into the amp.
For a Burns pick-up the amplification is just right with the schematic attached.
For lower output pick-up's maybe more amplification is needed.
I would start finding this extra amplification by splitting the 6BD11/Pentode cathode-resistor and bypassing only a section of it with an electrolytic cap.
Adjusting that pentodes operating point to the one shown proved optimal.
Actually a bit strange; I wouldn't have guessed but the Burns set it there.

With the Hammond OPT back in, the previous stated output re-appeared.
The amp's doing around 17-18Watt/8R with the shown B+ supply.
Since the 6AD10's are specified for Va Max = 275V they're running a bit on the hot side.
As with the max Anode/G2-dissipation being violated it doesn't make any difference to this tough GE Compactron.
Others (Like Sylvania and probably RCA) makes this valve as well.
I can't say if they're as happy in this construction as all I have on hand are the General Electric's.

The speakers used were my B&W 685 (8R/88dB SPL) Hi-Fi speakers.
Obviously with the Celestion 12" speaker promised, the amp will sound even better and probably more guitar-amp-like.

Odd resistor values in the schematic happened because of adding in series or parallel new resistors to what was already there adjusting operating conditions of the valves.
You may use the nearest standard value w/o problems as will I when the final amp-version is built.

For now I consider this project a success. It was a learning-process as I'm about to start building a 100W+ guitar-amp. Really heavy stuff that I'll post here under a new thread as it progresses. So far the chassis has been drawn in SolidWorks CAD. It needs to be laser-cut and bended. A project for the vacation-season

As the chassis and the cabinet is made for the Mini Gee-tah I'll post pictures in this thread. Don't exactly know when that will be as I depend on someone else for the 3D-drawing.

If you're interested in doing your own version of this amp. either for guitar (Schematic v2.5) or hi-fi (Schematic v2.4) use you will end up with a very in-expensive but very good performer.
Building with Compactrons CAN be successfull

Thank you for following this thread so far.

rgds,

/tri-comp

[tri-comp]

I decided to do som research into where this amp will go in 'Hi-Fi Mode'.
So, the Edcor CXPP30-MS-3.8K output XFMR went back in.
For Hi-Fi you should address frequency response as well as distortion and something like 20Hz to 20KHz or even much higher is a must at substantial power output.
I cared for the frequency responce in that I replaced the cathode-follower load-resistors with CCS' running 3,8mA into the cathode-follower; see new schematic.
The AC impedance of the CCS is very high around 22MOhm and it shows.
CCS Z-out = BC547B Hfe x MJE340 Hfe x Emitter-resistor = 220 x 70 x 1600 = 22,4MOhm.
With the CCS in place the output within Class AB (Not AB2 yet, that is grid-1 of output valves swing up to 0 volt and not higher) the output is flat from 17Hz to 35KHz at 10W/8Ohm.
Bringing the amp into AB2 territory with a g1 swing to +2V, yielding max. undistorted output = 12,25W/1KHz/8Ohm.
In AB2 the power decreases around 30KHz and distortion rises with skewing envelopes. Downwards response remains the same.
It isn't the AB2 drive that collapses in AB2 but rather the output valves. The cathode-follower happily swings g1 to +12V undistorted, but to take advantage of this, I will need higher B+ and not the 'measly' 300V I'm running now
Actually 300V is 25V more than valve-max spec.
See previous posts in this thread about what abuse the 6AD10 will cope with!
Do you feel lucky..?
The output valves are doing 32mA each quiescent, rising to 47mA at full AB2 output. Well within reason; the valves will last long with this configuration.
The Edcor is made for a 3,8Kohm anode-to-anode impedance but I tried different load-variants to see what suits the 6AD10 best. Using the 4-Ohm speaker-tab and loading it with 8-Ohm produces the highest output. That will rise the reflected a-to-a impedance to 7,6K which seems to make the 6AD10 happy.
You will notice from the schematic that the pentode voltage-amp is strapped for triode-mode. This, and at the same time using the original pick-up amp triode for pre-amplification yields the best performance. Sensitivity is a bit high; around 1Vpp input will make the amp VERY happy and you can easily get away with half of that.
And how does it sound?
VERY, VERY good. In Stereo it would be perfect. Bass you won't believe from such a small amp. Mids/vocals clean and highs brillant. You may get away with using cheaper output transformers as the Edcor is rated for 30Watt. A bit overkill for this amp.

rgds,

/tri-comp

[tri-comp]

Correction:
The CCS-Zener diode is a 6,2Vz, NOT 6,8Vz.
Actually it doesn't make much of a difference except it slightly changes the CCS-current and that's not very critical.

Today I had a chance to use the amp. with my DVD-player watching a movie.
PLENTY of volume for a regular home and fiersome bass when needed.
You wouldn't think so with 10W+ but it's there!

rgds,

/tri-comp

[grindrod]

Originally Posted by TriMan66

Just for my understanding, why did you chose to use a choke in a PI config for the HT, ie C-L-C. Are not more caps to give better smoothing less expensive than a choke.

The essence of a guitar amp is the way the clipping output stage interacts with the power supply, that is what gives most of the distinctive sound, also undersized output transformers play their part as well!! You don't want overall negative feedback either!

You build a guitar amp completely differently to a hifi amp.

[tri-comp]

You may be right about that ...at least in the past when you didn't much have any other choices for 'sound-shaping'.
Now my guitar playing friends have all kinds of electronic gadgets they install between the guitar and the amp to construct the required sound.
Having an output amp that doesn't colour the sound in the extreme seems preferable to them as it makes it easier to control the overall intended sound quality ...or lack thereof

My Mini Gee-Tah project was also done to investigate the possibilities of utilizing Compactron valves for this purpose and as it turned out for Hi-Fi purposes as well.
I wasn't disappointed. They are after all just $1.-/ea.

Anyone here did Compactrons and if so for what purpose and with what outcome?

rgds,

/tri-comp

[Herald1360]

That sounds like a sound engineering sort of solution to the problem. A kind of integrated "miking up" approach without the cost of keeping the appropriate classic amp happy and fed with expensive valves! High power HiFi is the easy bit nowadays.....

[tri-comp]

Well, I'm not exactly saying that I agree with them.
After all I still remember how a VOX AC30 sounded
I must realize however, that I'm getting so old that I'm really not able to discuss that tone with anyone else around, who also experienced it
So, I do what I'm asked... and try to make the best of it.
Best in sound quality/performance as well as economics.
It's always a compromise in real life!

rgds,

/tri-comp

[tri-comp]

MINI Gee-Tah for Hi-Fi revisited,

It became time to disassemble this proto-type ...or, to work some more on it
I've been enjoying Triode-PP for quite some time now ( https://www.vintage-radio.net/forum/...ad.php?t=67511 ) and I'm very, very satisfied with it's sound-quality. Soft, medium or full power doesn't really make any difference. There appears to be no harshness of tone until you actually hit the ceiling and heavy distortion kicks-in. That won't happen anywhere near my requirements for volume level on the above referenced amp, and that's a mere 12W/8-Ohm. All-in-all a very melodical amplifier and WHY DON'T MY MINI Gee-Tah AMP SOUND ANYTHING LIKE THAT!?
It does sound good, just not 'triode'.... hmmm...
Changing the phase-inverter operating-point; the input circuits, the CCS', the operating-points of the OP-valves... you name it, I tried it! Close, but no cigar Ultra Linear didn't change anything so what can be done? The sound, mainly vocals are harsh but nothing really shows up on a scope. Some phase-shift above 20KHz but even a 20KHz ramp and a ditto square passes QC imo. It just doesn't sound soft and pleasing like a triode-PP.
Then I finally had an idea; why not make it into a triode-PP? Output power would surely drop but probably enough would remain to still feature the very firm and full bass that was always present. Immediately after triode-strapping the 6AD10 OP-pentode it became evident separate bias trimpots are necessary. The valves are just not matched close enough to run on a common preset. No problem, ref. the new schematic.
The result, after rewiring the Edcor back to 3,8K a-to-a was astounding! My long sought-after triode-sound appeared AND power didn't dis-appear. Actually it does 18W/4-Ohm and a couple less in 8-Ohm. The CF-drive will swing G1 to +12V before the amp-output clips (not the CF-drive!) and way past that but then of course the sound becomes distorted.
More ideas lurk. Next I'll swap the 6BD11 pentode with triode-section-2 to make that the phase-inverter, still triode-strapped. Probably putting section-2 in the front will improve 'triode-sound'.

Rgds,

/tri-comp

[tri-comp]

Taking a more critical view on distortion I've come up with some corrected data for output.

Edcor output transformer wired for 3,8Kohm a-to-a

In class AB1 triodeconnected push-pull it produces 3,5W/8R, 18-23.000Hz and drops to about 2/3 at 45KHz
This is nothing to be celebrated and hardly worth the effort.
In Class-AB1 the CF/CCS isn't utilized and may even be removed, strapping the phase-inverter output directly to the output-valved through 100nF cap's not forgetting to modify the bias-circuit to connect directly with grid-1.

With AB2 and the CF/CCS-drive things improve.
With triode-strapped output-valves the CF is doing +12V at grid-1 before distortion sets in just beyond 12W/8R with same frequency response as above.
So 4 times the Class-AB1 output can be expected undistorted which is quite good for a few $1 scrap-valves!

I believe the CF/CCS drive may be used universally with any valve you wish to subject to AB2 within reason. The CF will work over a wide supply-range but of course there's a limit to what grid-current it's able to supply. in the case of 6AD10 where the built-in low-power triode-strapped pentode is used, it fits the also triode-strapped output-pentode, very well. In case of using the CF-drive with non-compactrons, I prefer a 5687 as CF. Overkill, perhaps but it will supply grid-current of any magnitude required (Heavy TX-valves excluded)

I have a bunch of NOS 807's that should perform very well in class AB2. One day I'll subject them to this CF/CCS circuit and see what happens. First I need to find some very heavy iron for the experiment
Since 6L6 is pretty much the same as 807 that too should sing in AB2.
Line Sweep-Valves may also be candidates but I'm not sure about this. Probably not a lot is gained with AB2ing them?
Frame Sweep-Valves should do well with AB2.

So many valves, so little time....

Attached is the not-so-cluttered schematic of the CF/CCS-drive and the triode-strapped push-pull output.

rgds,

/tri-comp

[Herald1360]

Originally Posted by tri-comp

Since 6L6 is pretty much the same as 807 that too should sing in AB2.

A pair of 6L6 in AB2 should manage just about 50W. I think you can squeeze about 80W out of a pair of 807 in zero bias class B.

[tri-comp]

There are many opinions on 807 in AB2.
STC is pretty much with you on the 80W but absolutely not in zero-bias!
I don't even think the 807 will do that in any case.
Sylvania is more generous; they promise 120W/AB2/ICAS with Va=750V/Vg2=300V.
I suppose that's very close to red-plating

I have some NOS Philips samples and they're probably a sturdy variant as with most Philips-valves.

Now, to do an 807 Stereo-PP AB2 will take what kind of wattage out of a power-supply generating Va = 650~750VDC and stabilized Vg2@300VDC? To be on the safe side including CF/Phase-inv + pre-circuits not much below 300W I take it, not counting filament-drain.
Heavy stuff!

I should be more experienced before taking on that sort of challenge

rgds,

/tri-comp

[Herald1360]

It seems I was a little understated on the zero bias bit, and it's a slightly unusual circuit.....

[tri-comp]

OK, I can go with 'slightly unusual' and I stand corrected!
Did you EVER see any other construction (...more like a contraption) like this?
A grid-swing of 555Volt peak isn't going to come from your ordinary split-load phase-inverter and spending a couple of 2A3 to do the job seems extravagance these days

Thanks for the input,

rgds,

/tri-comp

[tri-comp]

I just couldn't take the bread-boarding apart without first going for some more abuse.
The 6AD10's aren't easy to put into red-plate territory but today I succeded
Yesterday I received a gift; a pair of NOS Monacor TR-1120LC 100V transformers.
( http://monacor.co.uk/products/patech...er/vnr/170310/ )
The drawing you can download from Monacor doesn't say much about the xfmr, but here's what I learned:
The 100V primary is labelled: (30W / 60W / 90W / 100W / 120W / 0 / W)
The pin labelled 'W' is NOT a transformer coil connection but refers to the ground-lug under one of the bolts for the mounting-bracket.
To use this EI interleaved (Hence no Single-Ended use) xfmr for Push-Pull connect the B+ to '120W', one plate to '0' and the other to '30W'.
This makes for a symmetrical and balanced connection. Pretty well balanced I take it, I don't see any difference on the scope compaired with the Edcor's I used earlier.
Also Cathode-currents of the 6AD10's are balanced, indicating similar plate-loads.
With a load of 8-Ohm connected across the secondary pins '8' & '16', NOT '0' & '8' as you would expect, I'm able to extract ~12Watt sinus from roughly 70Hz to 70KHz where the response drops off quietly.
The B+ drops to around 285V and Ik rises to 85mA. That means one valve is doing 24W or exactly twice the spec's. Running for several minutes at 100Hz flat out finally provokes red-plating. Not bright-red, just barely noticable.
I would NEVER have guessed a 100V xfmr would be flat past 70KHz, but this one is.
Time to rethink from where you should get your next output-xfmr?
Obviously this xfmr is far overdimensioned for a pair of 6AD10s but it's very nice to know about it's capabilities for use in future bigger amp's.

rgds,

/tri-comp

[Ed_Dinning]

Hi Tri-comp, just a thought, why not de-laminate the transformer if it is not totally coated in varnish and re-assemble without interleaving the lams and an air gap of about 0.1mm (a sheet of thin paper). You might be surprised (or not) at how good it sounds on class A.

Ed

[tri-comp]

I wasn't quite telling the truth about receiving a pair of these xfmrs.
Actally I received 2 pairs
If not, I wouldn't even consider taking them apart and re-assemble as EI.
Before attempting anything so drastical I should have a plan for what to drive them with in class-A SE, and I don't.
I simply don't have a valve that'll fit the bill.
...other than maybe a triode-strapped 807. I have plenty of 807's.
Obviously an 807 triodestrapped will only challenge the xfmr to a small percentage. These transformers are HUGE (120W)
....and please, with the suggestion think about not driving me from my home

rgds,

/tri-comp

Edit: Eeh, I just remember that I did pick-up a quad of 6C33C's this spring when I went to Ukraine.
Only problem: You need to feed the filaments about 50Watt before they'll even consider singing.
THEN you'll throw another 70% of the supplied B+ power out as heat.
So, for 20 Watt or so of output, expect to heat the room with another 100Watt, minimum.
And that's just for one channel !

[Ed_Dinning]

Hi Tri-comp, not so huge really, but would probably sound OK on class A if driven by a
6V6 as it would be way down the flux density curve that an air gap would not be necessary.
807 would be a good choice.

On the positive side if it drives your neighbours from their home you could get some cheap houses!!

Ed