Yeah I didnt have a great multimeter at the time, but I think my measurements I made were consistent with what the other guy got at the time too.
Would have been a 1K hz sine wave i tested with.
Would have been a 1K hz sine wave i tested with.
I second your advisor. A 12AT7 LTP may provide more output voltage in comparison with a 12AX7 one, which is what you need to drive the relatively low gm 807's.
What was your voltage measuring equipment? Your scope? Perhaps you might have calculated with your Vss readings?
Best regards!
What I like of the 12AT7 in this specific case is that having less gain, you can raise the master volume of the preamp, that on the 2203 has usually a big benefit on the sound. If the first two 12ax7 are in the ballpark of the voltages I wrote you, you should be fine.
Are you using the standard 2203 output transformer running at a Raa of 1k7?
Are you using the standard 2203 output transformer running at a Raa of 1k7?
Actually PI gain with the 12AT7 (Av = 22) is not so much less than with a 12AX7 (AV = 26).
The main difference comes from the 807s having having 50% less gm and voltage gain compared to EL34s.
And of course the 4.7k NFB series resistor drops closed loop gain to a mere factor of 2.
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Thats what I thought, seems like 807's take a fair bit to drive as they are not a very sensitive tube.
I was going from my digital multimeter i had at the time which I doubt was a "true RMS" type meter.
I do have a better meter these days but have never made any measurements on this, will need to do this over the weekend and take a look.
I would not trust any readings from my scope as I can only make a guess from the division lines and make an estimate.
I had the OT custom wound, from memory it was 2K on the primary, need to dig up my notes as its been a while.
Needless to say this amp is quite unique, will be interested to confirm its actual output power, as I felt at the time it was putting out much more than 100W.
What I can say, is that ive had top musicians who have used it at a gig and told me it behaved and felt just like a marshall, so I guess that can only be a good thing.
My only real concern is the rather high anode voltage on the plate of the preamp tube, might be a good idea to tube roll and make more measurements.
When you play heavy notes, the blue glow inside the tube reacts and flashes quite bright as the tubes go under more load.
Its quite a sight to see.
Check the voltage of the power supply cap connected to V1. I prefer it in the 250 V range.
Check the voltage of the power supply cap connected to V2. I prefer it in the 280 V range.
At least for some guesstimation, you can read your scope as well. But you need to do the math right, which goes:
Pout = Vss²/(8*Zout)
807's in a Marshall-esque amplifier indeed isn't a bad idea. Remember that Marshall themselves once used the rather similar (with exception of the ratings, of course) 6550's in their amplifiers when reliable EL34's were hard to source. And also remember that the first Marshall's ever were true copies of the contemporary Fender Bassman, but with KT66's as finals instead of 5881's - both 6L6 derivatives as well as your 807's.
Good luck and have fun!
Remember, all those amps used a 12AX7 LTPI.
But I agree that this modified circuit needs the 12AT7.
Reason is not the 807s themselves (no current needed to drive them), but the low value (72k) power tube grid leaks loading the PI.
I guess the low value is a safety measure against thermal runaway.
V1 is about 372V and V2 is about 410V or so, see where I took the measurements on the schematic here:
Yeah, I dont think its a bad idea in itself, as you say the other tubes Marshall was using were essentially 6L6 derivatives.
Now I was just making some measurements on the amps output and I think my dummy load is putting me crook (i have one of those old wire wound resistors with a knob that slides a brush down it) I set the dummy load to 8 ohms on my multimeter, but I was getting a poor RMS voltage reading on my meter.
So I grabbed my spare 8 ohm PA speaker I have and put on my ear muffs, because its ear piercing when I put a tone through this thing.
Didnt take me long to reach 30V or so on the meter with the speaker connected and then I heard a pop from the speaker and its output dropped right off, I think only the tweeter is now making sound, its rated at over 200W, but something is not right, but nonetheless I was getting over 110W at 31V RMS if my math is correct.
Please consider that your speaker's impedance at your measuring frequency not necessarily is the same that is written on the badge, the nominal impedance.
Best regards!
Please, recheck them. On the schematic you have 372V on both V1 and V2, then 411V on V3.
Having RC in between them, it is not possible.
@nzoomed , to let you better understand, this is the impedance vs frequency plot of a nominally 16 Ohm guitar speaker:
ref: https://audioxpress.com/article/Test-Bench-Celestion-G12H-GreenBack-12-Guitar-Speaker-Reissue
Yes thats correct, Im not sure how far out it is with something only 8 ohms however, but i dont have any real way to measure my dummy loads impedance.
All i know is I see most people ive seen on their test bench are just hooking up large wirewounds with heatsinks as their dummy load in 8 or 4 ohm configurations.
Thats the measurements I was getting, but I will check again tomorrow. I will swap out for a 10K as well to measure the difference in voltage.
Regarding impedance, what frequency do manufacturers typically rate theirs at? Going by that chart 16 ohms is roughly at 500Hz. Looks like I could raise my dummy load up a bit more i guess.
It was a cheap speaker I tested it with, but i suspect the crossover couldnt handle it rather than the driver, but I will soon be able to find out.
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Most probably you read a value on a close point.
Looking at the schematic there's a 411V > 4k7 > 372V > 4k7 > 372V.
That would mean that V2 should drain 8,3 mA and V1 not at all.
I would more realistically expect 1,5 mA on V1 and 3 mA that on V2.
That would bring the voltages to:
411V > 4k7 > 389V > 4k7 > 381V
I would drop all voltages on V1 V2 and V3 to something like 250, 280 and 350V respectively.
Looking at the schematic there's a 411V > 4k7 > 372V > 4k7 > 372V.
That would mean that V2 should drain 8,3 mA and V1 not at all.
I would more realistically expect 1,5 mA on V1 and 3 mA that on V2.
That would bring the voltages to:
411V > 4k7 > 389V > 4k7 > 381V
I would drop all voltages on V1 V2 and V3 to something like 250, 280 and 350V respectively.
OK, i checked the measurements again, and I wasnt too far out.
It seems that the readings vary depending on the load of the amp, so i made sure all volume knobs were down all the way this time, If I turn up the volume about quarter or more the voltages below drop at least a good 20-25V.
The first resistor (which is only 1K) is also out of the equation here, so I have updated it as follows:
411V > 1K >400V > 4K7 > 378V > 4K7 > 372V
I just swapped out the 4K7 resistors for 10K again and checked the measurements and I get 411V >10K > 360V > 10K > 348V, so not much of a drop, that still needs 100V or so to drop down by to get to the recommended 250V on V1 you quoted earlier.
Now the plate voltages are still lower depending on the anode resistors, so I take it is more critical what the plate reads, rather then the HT supply feeding the anode resistor?
Am i likely to loose much output power on the amplifier if the preamp voltages get lowered too much?
Looks like I might have to drop these resistors down a fair bit more so will take a bit of experimenting by the looks.
If I replace the 1K with a 10K, will the other two in line need 10K as well or keep them at 4.7K?
I dont know why the old fella told me to drop it down to 1K, i always thought that was a bit on the low side. Actually, looking at that schematic, where I have got the 1K appears to have 2x10K in series going by whats written there.
Ive got 411V feeding this out of the choke filter, cant remember what voltage it would have been if it was a marshall amp in this place, but from memory it would have been about 380V.