single ended output, no gain?

vixo

Member
2016-01-25 10:03 am
Im trying to troubleshoot a low power, single ended output guitar amplifier (one EL34). The amplifier suddenly lost a lot of volume. I think I have tracked the problem down to the output stage - the plate voltage shows there is almost no gain between the grid and the anode. If I plot the load line, it shows - if i am not mistaken - that the plate should swing hundreds of volts! I have tried replacing the EL34 - no change. I measured the reflected impedance of the transformer and it's around 3.5Kohm and it performs fine when tested with a variac. The components around the EL34 measure fine, although I have not replaced them.

I'm scratching my head and wondering why this might be - can anyone confirm (or disconfirm!) that I should be seeing a much larger voltage on the plate, or provide me with a suggestion of where to look next? Here are some measurements I have made of the EL34 -

all measurements referenced to ground
pin 1/8 = 23V DC
pin 3 = 346V DC
pin 4 = 327V DC
pin 5 = 0.1V DC

with 9.7V AC on pin 5, I get 12.41V AC on the anode! almost no gain!
 

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With some 15V RMS drive at the grid, you should get some 200V RMS at the plate (be careful when measuring that) but, more importamnt, you should get some 8 V RMS at tge speaker output (8 W RMS into 8 ohms) or thereabouts.
So test that first.

You *might* have a shorted OT what actually does not mean it's a block of molten copper with 0.01 ohms impedance but that (often because of using it without a speaker connected) , some very high voltage sparks burned through the wire enamel insulation and shorted some turns together.

Now the transformer still "works" ... sort of ... but those shorted turns absorb a lot of the power, amp sounds weak, compare it to driving with the handbrake applied.

Search for R.G Keen's shorted transformer tester, a very simple and clever circuit, but you'll need to get a neon bulb somewhere (worst case cannibalize an old neon lamp indicator screwdriver or a dead electrical coffee maker, etc.)
 

vixo

Member
2016-01-25 10:03 am
With some 15V RMS drive at the grid, you should get some 200V RMS at the plate (be careful when measuring that) but, more importamnt, you should get some 8 V RMS at the speaker output (8W RMS into 8 ohms) or thereabouts.
So test that first.

You *might* have a shorted OT what actually does not mean it's a block of molten copper with 0.01 ohms impedance but that (often because of using it without a speaker connected) , some very high voltage sparks burned through the wire enamel insulation and shorted some turns together.

Now the transformer still "works" ... sort of ... but those shorted turns absorb a lot of the power, amp sounds weak, compare it to driving with the handbrake applied.

Search for R.G Keen's shorted transformer tester, a very simple and clever circuit, but you'll need to get a neon bulb somewhere (worst case cannibalize an old neon lamp indicator screwdriver or a dead electrical coffee maker, etc.)

ok, I'll endeavour to double check that but I *think* my OT is fine - the reflected impedance checks out fine and measures on a variac as a 1:25 ratio, which if I had 200V on the primary would give me 8V on the secondary!

With 15 VRMS on the grid I only get 15.5V RMS on the anode!

Have you connected a 100k (max 250 kohm) resistor between pin 5 (grid) to ground to get a reference?
The 56k in series to grid is too high; 1K is better.
Also the 56K in series to g2; 1 K 3w max.

thanks Waltube, that's good advice, but seeming as the amp was working fine previously, I want to find the reason it doesn't work before modifying it...
 
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This thread is a continuation of the original:http://www.diyaudio.com/forums/instruments-amps/285844-advice-troubleshooting.html

Vico said the 56k was a typo. I highly suspect the opt has been run without a speaker attached. Someone please explain why his opt primary passes dc just fine, but the tube ain't swinging the primary (much less the secondary) like it should. Cuz post #3 wasn't sufficient. We don't want to buy another opt. Without at least testing it under properly loaded conditions.
 
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Hi Guys

The screen-stop should be a 1k-5W not 56k. This will severely limit output power but does not explain the change in performance claimed.

An open coil can often test okay with applied voltage but then crap out under a load test. Shorted or open coils both mean the OT is bad. It is a simple enough matter to tack in even a small PT to try as an OT to verify that a different part there will allow the circuit to work.

Replacement OTs for guitar amps like this one are quite inexpensive.

Have fun
 

Parafeed813

Member
2012-08-15 8:24 am
Vixo, in the other thread you said the HT drops when driving the EL34 hard.
This means that the EL34 is working hard as well. You probably see an increase in cathode voltage to reflect the increase in anode current.
So that would leave the path after that: OPT, cabling, speaker jack, load (speaker or resistor).
How did you do the variac test of the OPT? Still in the amp or taken out? Including load resistor?