E80CC LTP PI instead of ECC83

[SoundCity85]

Hello,

I'm working on my own design of a SLO100 preamp into a Sound City amp. No need for a clean channel, I don't mind it'll be a one trick pony. I go for a two channel layout like a Hiwatt, where the channels are mixed after the first gain stage. Low and Hi input, so I can use the Hiwatt 4 hole face plate for this amp. I will wire V1 to be used with ECC808, but have adapters to also fit them with ECC83 (not the other way around).

I worked my way up the PI, and got the idea here for using the E80CC for PI, because I have 4 of these tubes at hand and like to try them for this build. How do I need to modify the schematic to fit in the E80CC?

Cheers

[pdf64]

https://tubedata.altanatubes.com.br/sheets/009/e/E80CC.pdf

When asking such a query, it would be beneficial to provide basic info, such as the above, and the schematic you want to base the power amp on (which is it to be, SLO or sound city?).

[SoundCity85]

Hello,

Besides expansion of the first gain stage this will be more or less it:

https://robrobinette.com/images/Guitar/Soldano/Simplified_SLO-100_Schematic.png

[SoundCity85]

This is the power section

I basically make the cut at the coupling caps. This worked for both 2204 and Hiwatt preamp into this power section.

[SoundCity85]

The power amp will be the Sound City 120, I won't make any changes there. I basically take a L120, gut the preamp up to the coupling caps of the PI, and take it from there

[pdf64]

Just as well you made that very clear, as I would normally understand everything within a global negative feedback NFB loop to be covered by ‘power amp’.
Whereas you only seem to mean the power valves.

Taking the NFB feed from the 100V output is unusual. Due to all its additional windings, it’s likely to have poorer coupling, so the waveform there may differ a bit in phase and freq response to that at the regular outputs.

[SoundCity85]

That's a good point. I can easily rewire that to the 8 ohm tab and adjust the resistor value.

And what about the implementation of the E80CC?

[pdf64]

Its characteristics look very different to those of an ECC83. So if you want the same performance from an LTP as that of the SLO, I suspect it’s not a good choice.

[SoundCity85]

But since I don't own a SLO, I have no reference point  Also I'm building it British style with mustard caps and EL34 so in any case there will be sonic differences.

My guess is that the OD is being created and layered before the tone stack. Will this change in PI only result in a volume drop due to lower Mu? Is it then a matter of volume loss that can be partially compensated by just crancking the master volume a little louder  ? Or will it feed the power tubes with a signal that is not appropriate? I need to educate myself a bit more on impedance of AC signals...

If you think generally it is a bad idea to consider, then I will trust your judgement and abandon this experiment :-)

[pdf64]

It may work ok with all values the same, just swapped into the regular LTP circuit. Doubling or tripling the value of the 470 cathode bias resistor may be beneficial though. But whatever, the gain will be significantly lower, which may affect the degree of presence boost available, and even with the master up full, the amp may not reach full power. 

Other folk around here may be able to advise better, I’m improvising somewhat here 

[2deaf]

Even with a LTPI gain of 9, there is plenty of open loop gain due to the high EL34 transconductance and high OT gain.

I would start with 47K plate resistors, 1K cathode resistor, 10K tail resistor, and a 4.7K/100K NFB voltage divider off of the 16 ohm OT tap.  Plenty of voltage swing on paper for EL34's with an E80CC LTPI.

[SoundCity85]

Thank you for your input 

[SoundCity85]

Instead of the SLO, I'm now rebuilding the SC120 as a cascaded high gain amp. I want to try the E80CC instead of the ECC81. This is the schematic of the LTP. How would conversion to E80CC be done?

Not on the picture: NFB value is now 22k from the 100V tap.

Also: I measured Mu of these tubes, and they are matched at 32 +- 0.5

[pdf64]

… NFB value is now 22k from the 100V tap.
…

It may help to think of that tap as being intended for an 83ohm load.
ie P = V2 / R, so
120 = 100 x 100 / R, so
R = 10,000 / 120 = 83

[SoundCity85]

Thanks. 22k seems like a rather low value then, if I swap it to the 16 ohm tab it converts to 8-10k ohm.

[SoundCity85]

It seems to resemble the most like a DC coupled LTP with addition of NFB. I guess I'll just start with ECC81, see how the amps sounds, drop in the E80CC, does it sound better or worse, and the same goes for the value of the NFB resistor. Tune by ear. My knowledge is too limited to do something better