A friend brought this amp to me, saying that the amp is breaking up at low volume levels. With the master volume at 50%, he said he could only go up to 50% on the clean channel before break up. I cannot get the schematic or any technical info about this amp. I remember seeing somewhere on the net that this amp is rated 15watts with an 8in 8 ohms speaker. It has a single 12AX7 and 2 6L6GC tubes. The preamp is solid state. There is a pot on the amp for bias. Upon checking the bias, I get 39.44mA on one tube and 33.90mA on the other at 427V plate voltage. So one tube is dissipating16.84 watts and the other 14.48 watts, with a total of 31.32watts. This is double the rated power and can this be the reason for the early breakup. Is this bias setup safe and acceptable for this amp. Please assist.
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It seems unlikely to me that the amplifier output is only 15 Watt, while having a pair of 6L6GC's in push-pull at a plate voltage of 427 V. Looking at the datasheet, something like 30 to 40 Watt seems more likely. The dissipation is not the problem (well under the limit of 6L6GC's), and maybe even too low (biased a bit 'cold'): http://frank.yueksel.org/sheets/049/6/6L6GC.pdf
OK, the power rating is within the range of the 6L6s, but what about the output transformer. Can it handle. If I change the bias and set it at the recommended 70% rating, that is approx 21watts per tube, should it be OK with the output transformer.
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Sorry, the tubes used here are 6CA7EH and not 6L6GC. I was told that they were originally fitted with 6L6GCs. Do they match.
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NO, they do not. The bias needs are different, but especially, make sure pins 1 and 8 are wired together. In the 6L6 the suppressor grid is bound to the cathode, but in EL34 they are separate. You need your suppressor connected to the cathode. (Yes, there are alternatives, but this is just a simple guitar amp)
On this amp, both pin8(cathode) are tied to ground and pin1 are not used. Does it mean that this amp is wired for EL34s or 6CA7s. I do not know what are the original tubes meant for this amp. When it came to me, it was fitted with 6CA7s. Another thing I realised is that the signal from the preamp(solid state) distorts when I set the channel volume above 50% with the master at 50%. I checked this using a scope connected to the signal from the preamp to the power amp. The waveform clips above 50% with the master and all the tone controls at 50%. Is this normal.
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I'm puzzled.
Your first question is already answered clearly by Enzo in post #6. And when you type "6CA7" in a search engine, it's pinout is readily found. The amplifier is clearly meant for 6L6GC's.
Your second question: How do you think we could know without knowing what voltage the input signal is? But in almost every guitar amplifier fitted with channel volume + master volume, you can let the input signal distort by crancking up the channel volume (while still being able to control the total volume of this distorted signal with the master volume).
I can hardly believe that I'm writing this to a forum member who often writes about musical amplifiers on his repair bench.
Your first question is already answered clearly by Enzo in post #6. And when you type "6CA7" in a search engine, it's pinout is readily found. The amplifier is clearly meant for 6L6GC's.
Your second question: How do you think we could know without knowing what voltage the input signal is? But in almost every guitar amplifier fitted with channel volume + master volume, you can let the input signal distort by crancking up the channel volume (while still being able to control the total volume of this distorted signal with the master volume).
I can hardly believe that I'm writing this to a forum member who often writes about musical amplifiers on his repair bench.
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That means nothing.
Whatbdoes it actualy measure at clipping?
What does the waveform look like? (Post a screen image).
And why setting master volume to 50%?
Set it up to 100% and remeasure.
An 8" speaker in a 2x6L6 amplifier?
VERY unlikely.
So the 12AX7 is the phase inverter; power amp should be "Fenderish", Japanese way back then used to more or less copy successful US amps.
The SS preamp would be a way to keep cost down.
Quite reasonable values, I see no problem there.
WHAT rated power?
I hope you don´t mean the supposed 15W output rating.
Which early breakup?
Have we measured actual power output already?
The numbers you post seem safe.
Thanks for posting.
That is certainly a WEIRD amplifier but power stage is, as I suspected, 100% Fender.
And it states 45W power out, what´s expected with a couple 6L6 and voltages shown, don´t see why carlos raj´s would be any different.
Talking about the power amp; preamp can assume many forms but that won´t affect power output.
Are we still discussing the amplifier fitted with 6CA7's instead of 6L6GC's? And with the suppressor grids (or probably better: the beam forming plates) not connected to the cathodes, so floating?
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