Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: theanalogkid on April 10, 2019, 06:51:06 pm

Title: heptode amps
Post by: theanalogkid on April 10, 2019, 06:51:06 pm

Has anyone ever encountered this amp? I think it was in the 2nd edition of glass audio magazine. It uses an ECH81 for a preamp/phase inverter. It's variable from SE to PP. I've included the main schematic and the whole article as a PDF.

Title: Re: heptode amps
Post by: jjasilli on April 11, 2019, 09:22:50 am

Very cool concept!

Title: Re: heptode amps
Post by: PRR on April 11, 2019, 08:17:21 pm

Hard to believe 10K under the power tube cathodes.

Title: Re: heptode amps
Post by: uki on April 11, 2019, 08:22:26 pm

Quote from: PRR on April 11, 2019, 08:17:21 pm

Hard to believe 10K under the power tube cathodes.

Not expected values! Would you explain please?

Title: Re: heptode amps
Post by: DummyLoad on April 11, 2019, 08:23:13 pm

Quote from: PRR on April 11, 2019, 08:17:21 pm

Hard to believe 10K under the power tube cathodes.

hellofa cathode followers with nothing following?  :BangHead:


10R Is probably what is wanted. si?


--pete

Title: Re: heptode amps
Post by: VMS on April 12, 2019, 03:52:24 am

I'm guessing that ECH81 could also be used for mixing two channels or feeding dry signal to one grid and reverb signal to the more sensitive grid.

Title: Re: heptode amps
Post by: kagliostro on April 12, 2019, 04:15:01 am

There is something build that way

one old Australian amp

at home I've a scheme

Franco

EDIT: OK, not exactly what you are saying

Here are the Playmaster 102 and 103 schematics the 6BE6 is used to "mix" signal and vibrato

(https://i.imgur.com/DpT36N4.jpg)

(https://i.imgur.com/CRCzf1W.jpg)

Title: Re: heptode amps
Post by: VMS on April 13, 2019, 05:21:15 am

Yes, not quite the same but interesting looking circuits, thanks for sharing

Title: Re: heptode amps
Post by: theanalogkid on April 13, 2019, 09:31:06 am

I would like a more in depth analysis of the circuit. Or at least a more in depth analysis the in the authors article. I understand a lot of the amp, but still have questions. The 10k cathode resistor on the power tubes. For example. I've never heard of such a high value. Your supposed to be able to use any pentodes in the circuit, but I don't fully understand the math to get the right values for other tube types. The power supply shows 460 volts for the B+ and the main schematic shows 480 volts for the B+. Typo? Any and all information is appreciated.

Title: Re: heptode amps
Post by: VMS on April 13, 2019, 10:21:51 am

As PRR and dummyload pointed, R14 and R15 resistors are 10R resistors and not 10K resistors.


Article says:

Quote

set the bias to 40mA (0.4V over R14 and R15)