Harmony H306a low output volume

[davohilts]

I've been working on this one off and on for a while and thought I'd give it another try. It has very low output volume using several speakers without any real change. I also have switched out tubes both preamp and power and get the same result. I have replace the filter caps and the cathode bypass cap along with the power and cathode resistors. When I first got the amp it had been modified to something I could not make heads or tails of, so using the schematic I've rebuilt it using many new components as the ones that came with the amp were not original nor correct.
The transformers look original.
I measure about 8.6 watts per output tube and have 345V on the plates of the 6V6's, 280V on the grids, and 17.4 volts across the cathode.
V1 has 119V/117V on the plates, .87V/.89V on the cathodes.
This is an interesting circuit in how the signal moves thru the amp and the schematic shows the B+ of the cathode on the power tubes being tapped and added on the AC of the filaments. I made a virtual center tap using a pair of 100 ohm resistors tied into each leg of the filaments and ran the B+ into that.
I am wondering if I have a bad output transformer? I don't have a working scope so I'm unsure how to measure my output from the transformer.
Here is the schematic.
Thanks for any help.
Dave

harmony_h306a.pdf

[davohilts]

when I say very low, what I mean is fully cranked it is not as loud as my GA5t on 1/2 volume.

[The Dude]

I would insert a test tone and measure AC with your DVM if you don't have a scope. Follow the signal chain. It should become apparent where there is a gain issue.

[Randall]

the schematic shows the B+ of the cathode on the power tubes being tapped and added on the AC of the filaments.
[ATTACH]32541[/ATTACH]

Um.. not exactly. Follow the center tap from the output transformer back to the rectifier tube V6, that's your B+.

[davohilts]

I would insert a test tone and measure AC with your DVM if you don't have a scope. Follow the signal chain. It should become apparent where there is a gain issue.

I have a signal app on my phone and can run a 1.0 kHz signal into the input of channel one, I measure 4.5 VAC across the dummy load resistor with the tone knob set to the middle between bass and treble.
As I follow the signal thru the amp it just keeps getting louder. The signal at pin 3 of the 6V6's is nice and strong and sounds about the same on the positive side of the OT.

so, if I'm doing this right the amp is putting out 4.5 x 4.5/8ohms = 2.53 watts

[davohilts]

Um.. not exactly. Follow the center tap from the output transformer back to the rectifier tube V6, that's your B+.

The schematic shows two places where there are connections between areas of the schematic. They are labeled 'A' and 'B' on the schematic. 'A' connects the center tab of the modulation pot on the tremolo with the connection of R22/R23 on the grids of the power tubes and the footswitch. 'B' shows the connection of the positive side (what I called B+ around 17.5 V) to one leg of the filament winding. That is what I meant.

[g1]

What they are calling B is not what we normally call B+. Here, B is the power tube cathode voltage. What you did with the 100R's going from filaments to B sounds correct.
Make sure the OT center tap is not wired up as shown in the schematic. I don't think that will work.
Edit: sorry, no problem with that wiring, I wasn't seeing the connection at top of C14.

[davohilts]

yes I know, it is unfortunate that it is labeled 'B'. I was interested in the circuit because I've added the B+ from the cathode bias resistor back into the filament centertap (or virtual in this case) with great results in quieting hum. It was interesting to see it actually on a schematic!
I'm not sure what the heck they are showing on the schematic but I grounded the high voltage B+ center tap to the same lug I grounded the 3 wire power cord.

[The Dude]

What is the signal level output by your phone? We need to make sure it's capable of driving the amp to full output power. Can you post readings from the power tube grids and plates for comparison? You should have quite a bit of gain there.

[The Dude]

I'm not sure what the heck they are showing on the schematic but I grounded the high voltage B+ center tap to the same lug I grounded the 3 wire power cord.

g-one was referring to the output transformer not the power transformer. I'm pretty sure the center tap of the OT is not grounded.

[davohilts]

What is the signal level output by your phone? We need to make sure it's capable of driving the amp to full output power. Can you post readings from the power tube grids and plates for comparison? You should have quite a bit of gain there.

here is where I'm in trouble. My phone app allows me to select frequency which I have set at 1 kHz and volume which goes from 0 dB to -60 dB. I had the phone set to -20 dB which is its default. I don't really know any more that that. I also need some clarification on what you mean when you say 'post readings from the power tube grids and plates' do you mean something other than voltages? Sorry about being so lame here.

[The Dude]

First off, you're not being lame. There's no such thing as a stupid question. Well, unless you read the blonde jokes in that other thread.....
You should be able to measure the voltage level out of your phone just as any other external circuit. This will tell us what level we are starting with (in AC volts).
Then, yes, measure the signal level (AC volts) at the grids of the output tubes and again at the plates so that we can calculate how much gain is there.

[The Dude]

On a side note: Since this is in a way a "new build" with you having to put the amp back to stock, you may also want to check out RG's debugging page.

Tube Amplifier Debugging Page

[davohilts]

With my phone set I test AC output at .04 VAC at 1 kHz so this is what is input to the amp.
Measurement at dummy load at OT is 4.5 VAC
Other Measurements:
Grid V3 = .59 VAC Plate of V3 = 61.5 VAC
Grid V4 = .57 VAC Plate of V4 = 58.5 VAC
This confuses me because of how much larger the plates are than the 4.5 VAC at the OT.

[The Dude]

Plenty of output tube gain and voltage. You said you checked with different speakers. Have you checked speaker wiring/cable? Not shown in the schematic, but does this amp have external and internal speaker jacks and maybe you're plugged into the wrong one? It seems to me the problem is right there at the output- maybe a bad OT.