Yet another 5E3

[kaiowas]

Hi!

First post here after lots of reading in this forum.

I had 3 transformers (2 OT and 1 PT) from a Fisher X-100. It was original operating with a couple of EL84 each OT. So i guessed that the primary impedance should be around 8800ohms or so and that they would be around 20Watts which is fine with a couple of 6v6.

To clarify I made a "testboard" for tubes (which is a piece of wood with terminals on), I'll take some pictures of it and post them later.

In the pdf file you'll find a table I made in excel for the measurements and some calculations (red numbers). If someone wants the excel file I can send it, I'm posting a pdf because is not possible to attach excel files.

I know I have a hight voltage (439V) out from the PT so I put a 470ohms (5 watts doesn't seem to be enough, as the resistor is getting hot, I guess 10 Watts should do it) cathode resistor for the couple of 6v6 to limit a little bit the bias current. But calculated power is around 12 Watts, which is fine (if my calculations are correct). Should I try to low a little bit the voltages? Or is power what matters the most?.

I'm also attaching some pictures of the impedance measurement of the OTs with the calculated N1/N2 for each of the output windings. I'm a little confused about this subject, so any interpretations of the measurements are welcome. (I measured the impedance with Loudspeaker Lab and then I exported the results to a MATLAB script to calculate N1/N2). What other information can I calculate from the impedance and N1/N2 measurements?.

For the OTs measurement I put a 10k ohms resistor in the primary and measured the impedance on each output winding.

Thanks for your attention

Kaiowas

[MWJB]

Your B+ is just about in the ball park with a 470ohm resistor, yes a little high but not rediculously so. First I'd try 330ohms at the 6V6 cathodes. I would expect your cathode resistor to get hot..at 5 or 10W. Keep the bypass cap physically apart from the resistor.

You are measuring plate dissipation, not power output. If you really have 11-12W dissipation now, you could certainly stand to decrease the cathode resistor. 13-14W plate dissipation should be fine.

To determine OT primary Z, apply 0.5VAC (measured) to the OT secondaries. Record the voltage developed end to end at the primaries. Divide that voltage by 0.5, then square it, then multiply by 8ohms...you should end up with 8-9K?

[kaiowas]

Thank you very much for your post MWJB. I'll try lowering a little bit the cathode resistor.
I forgot to tell that I'm having some problems with the volume pot. At high levels the amp starts to make some kind of "ring modulation" of the input signal. Very annoying. The other thing about the same pot is that is passes from total silence to a considerably high level with just a little turn. Any ideas?. I guess this could be a consecuence of the high voltages in all tubes with respect to the original design. Just a guess.

[MWJB]

Your ringing could be due to grounding/layout issues, shorten and or shield your grid wires. How does your grounding scheme differ from a Fender built 5E3? The scheme at www.hoffmanamps.com has always worked for me, but others also have success grounding to the brass buss plate. If you have any pics that might be useful for prompting suggestions?

If you compare the 5E3/5E4/5E5 volume pots to most other amps you'll see that they are wired differently to usual. They do tend to ramp up quicker than pots wired as traditional voltage dividers.