Epiphone EA-28RVT low power?

[neskor]

I just got old Epiphone EA-28RVT from '60.
Basically same amp as Gibson Falcon GA-19RVT.
Sounds good but have early break up.
Measured all voltages on the tubes from the schematic in everything looks fine. Also checked all signal capacitors for leakage.
All electrolytic capacitors replaced and all resistors checked.
Tried some signal measurments and something is wrong here.
Input signal is 220mV, on the output I'm getting only 7VAC before clipping sine wave. On 8R load that is around 6W.
It should be at least 12-15W for the pair of 6V6GT.
Tried different tubes and new 18W output transformer but I'm getting the same results.
Original output transformer is around 8K on primary and secondary 8R. Transfer ratio is 32:1.
Tried scoping the signal to see where it distort and I'm getting on the grid of output tubes only 12VAC before clipping.
Tried the same thing but without output tubes and I'm getting around 22-24VAC (on the grid pin of output tubes) before clipping, but when I insert the output tubes signal again starts clipping on 12VAC.
Also tried without reverb and tremolo circuit but getting the same results.
Is this normal behavior for this kind of amp and output transformer?

[TUBEDUDE]

Does the bias on the grids remain stable? And the plate voltage doesn't drop excessively on conduction?

[neskor]

this is a cathode biased amp
I will check later today once more is there any difference in anode voltages before and after the clipping, but I think from my memory it was couple of volts down, nothing major

[neskor]

I just checked:
Anode voltage is 290V and Cathode voltage is 19V, without any signal on input
with the signal at input, amp starts to clip on 7.6 VAC on output (8R load), Anode voltage drop to 278.5V and Cathode voltage rise to 22.3V

[Stevem]

With the extra heater load of having the output tubes in what changes if any take place to the heater voltage you read?

[neskor]

Heater voltages are constant 6.17VAC, no matter if the signal is clipping or not.

[Stevem]

What do you have for wall outlet voltage if your in the US

I would think that with today’s voltages of above 117 in many locals that you would have more then 6.17 VAC showing up for the filaments.

If you lift one end of C4 when the amp is into clipping do any improvements take place?

Any changes take place when you clip out one end of R15?

[Stevem]

Something is really eating at me about this!

With 300 AC on a 5Y3 you should have over 320 VDC available.
I can’t see why you have only 300 VAC when a GA-6 with two 6V6s has 320 VAC on its 5Y3 and over 330 VDC on the output tube plates.

Do you have a known strong good testing 5Y3 to put in, or even a 5AR4 if your first PS filter will take the 390 or so VDC that should show up?

You did not post up anything about having tested out your tubes you have in there, but I really think you simply have a dead dog of a 5Y3 in there .

[neskor]

What do you have for wall outlet voltage if your in the US
I would think that with today’s voltages of above 117 in many locals that you would have more then 6.17 VAC showing up for the filaments.
If you lift one end of C4 when the amp is into clipping do any improvements take place?
Any changes take place when you clip out one end of R15?

I'm in EU and using step down transformer 240V/120V
As I mentioned before, I tried with new output transformer and no change, so no R15 or R16 on the secondary of the new output transformer
C4 in or out, doesn't make a change

[neskor]

With 300 AC on a 5Y3 you should have over 320 VDC available.
I can’t see why you have only 300 VAC when a GA-6 with two 6V6s has 320 VAC on its 5Y3 and over 330 VDC on the output tube plates.
Do you have a known strong good testing 5Y3 to put in, or even a 5AR4 if your first PS filter will take the 390 or so VDC that should show up?
You did not post up anything about having tested out your tubes you have in there, but I really think you simply have a dead dog of a 5Y3 in there .

I tested all tubes except 5Y3, i will try to replace that tube.
If you check voltages on the Epiphone schematic you will see that voltage after 5Y3 should be 300V

[neskor]

Replaced that old 5Y3 with a new one and no change, ramp up the voltage on variac to get 310 on anodes of output tubes but no real change, now is clipping at 8V on 8R load.
Also tried to connect signal directly to grids of output tubes, to bypass the whole preamp, but no change.

[Stevem]

To me in terms of your chosen drive signal level into this amp I think it’s too hot.

No guitars in 1960 where any where capable of outputting .220mv.

I test with .150mv to .180mv at most, and .150 mv 95% of the time is more then enough to drive a amp to peak output if all the tubes are in good average shape.

[didit]

Hello —

Measuring 20V across 270 Ohm cathode resistor suggests try a tweak to your bias. Put in 330 Ohm and see where clean output sits. Doubtful you'll see it anywhere near 12W however. Also agree with comment that your test signal is overly hot.

Best ..

[Stevem]

I did some math on your output stage which I think is correct.
Please others check me out on this if you will.

By assuming a 8K transformer and a 8 ohm secondary load here’s what I get when you’re looking for 15 watts of power.

You need to develop 1.88 amps on the secondary into the 8 ohm tap to drive the speaker with.

You need 10.95 vac on the secondary into that 8 ohm tap to drive the speaker with.

Current into the primary of the OT to drive the 8 ohm tap to 15 watts is 118ma.

The voltage needed on the plates is 368 to keep the tubes out of cut off.

For a guitar amp this voltage might be about 10% less since where not talking about audiofile needs.

Since your output cathodes are idling at 19 volts you really only have 271 volts the tube plates to work with, and this to me is why you are only producing 7 watts or so of output power.

[Stevem]

Also I would like to note with the OP that his amp came with a CTS brand 12” driver.

The Gibson model amp he refers to came fitted with a Jensen C12R driver .
The C12R and its Jensen Alinco magnet brother ( P12R ) where only rated at 14 watts as my vintage Jensen info chart here shows.

These CTS and Jensen drivers would never have been used in an average model amp that could truly output a solid 15 watts of rms power from what I have seen in the many decades of working on these amps.

Manufacturers back then never thought for a second that there amps would be played flat out wide open and push the speakers they where made with into burning up there voice coils.

My own 1954 Gibson GA-77 for example was rated at 25 watts of rms power and 35 watts peak.

It came with a Jensen top of the line for back then P15N driver which as this chart shows was rated at 25 watts.

The best level of output power I have ever seen from it with all new filters of a higher uf value then original, power supply and output stage resistors new ( and of 2 times the needed wattage rating ) very strong testing tubes including the output tubes of course ( especially the 5V4 rectifier ) and running on todays higher wall voltage level produced 23 watts of rms power and peak of 31 into a 8.2 ohm resistive load.

So there you have it just for the record.




Without doing more math I will take a guess that even if you had 300 volts on the output tube plates and the amp was fixed bias and not cathode biased you would still not see more then some 11 watts of output .

Also last but by far not least please post up what your getting for AC voltage onto the rectifier from the PT both with the output tubes out of the amp and then with the tubes in the amp.

I am curious as to what percentage of regulation your PT has even just between basically no load and at normal idle no less between normal idle and max wattage.

At this point to me the only thing left that might be making for your low power issue ( if it really is a problem) is simply your power transformer capability’s .