Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dtbradio on June 11, 2023, 08:04:39 pm

Title: Identifying Oddball Tube Amp Chassis
Post by: dtbradio on June 11, 2023, 08:04:39 pm
Hi All!

I picked up this amp chassis for $20 today. No tubes or documentation included. The only markings are a stamped circle with "32E" inside. The over-all circuit appears to be push-pull using some kind of octal power tube like 6v6, 6l6, etc (pin 1 unused, pin 3 [plate] tied to one side of opt primary, pins 2 and 7 heater). Here's where the odd comes in. The screen grid pins (pins 4) are tied to the cathodes (pins 8), which are tied to what appears to be a filter capacitor positive voltage node. Both grids are tied together, and go to the 9-pin octal style socket. The power supply uses solid-state rectifiers, and appears normal enough. There appears to be both main B+ and a single-diode negative voltage source, and there is what looks like a bias POT with no guts which is NOT tied to the power tube grids. I can't figure out what exactly its purpose was. There doesn't appear to be any specific grounding scheme, as some grounds go to chassis, while others tie together in a semi-star style. My gut instinct is to just gut it and start a build from scratch. However, if this is a bit of a rare amp configuration worth a possible resurrection, I'd consider that undertaking.

Any help/input appreciated!
Title: Re: Identifying Oddball Tube Amp CHassis
Post by: sluckey on June 11, 2023, 10:53:14 pm
The output tubes are 7591s. Pinout is different from 6L6. Here's a link to my amp schematic that uses 7591s...

http://sluckeyamps.com/hammond_2/AO-63.pdf
Title: Re: Identifying Oddball Tube Amp CHassis
Post by: kagliostro on June 12, 2023, 12:38:23 am
(https://el34world.com/Forum/index.php?action=dlattach;topic=30429.0;attach=106695;image)

Merveilleuse  :bump1:

Franco
Title: Re: Identifying Oddball Tube Amp Chassis
Post by: dtbradio on June 12, 2023, 05:56:16 am
Quote from: kagliostro on June 12, 2023, 12:38:23 am
(https://el34world.com/Forum/index.php?action=dlattach;topic=30429.0;attach=106695;image)

Merveilleuse  :bump1:

Franco

Yes indeed!!! My plan is to squeeze those connections between my fingers while powering up to make sure they make good contact. I'll put my toe in the wall socket to provide AC mains power to those connections. It'll be fine! I have callouses to protect me, LOL!

Quote from: sluckey on June 11, 2023, 10:53:14 pm
The output tubes are 7591s. Pinout is different from 6L6. Here's a link to my amp schematic that uses 7591s...

http://sluckeyamps.com/hammond_2/AO-63.pdf

Yes, this was confirmed on another forum as well. I'd never even heard of the 7591 until this morning when I read the other forum poster's reply. After looking up the 7591 spec sheet, I have to agree that its the most likely tube to fit this setup. That thread is at https://audiokarma.org/forums/index.php?threads/identification-of-oddball-p-p-tube-amp-chassis.1032663/ for reference.
Title: Re: Identifying Oddball Tube Amp Chassis
Post by: RadioComm on June 12, 2023, 02:56:29 pm
Just read 6DZ7 post from the other forum: "Go back and ask the seller if they'd take it back for $10. Otherwise, just strip the .... na. Tape $10 to it and leave it in their driveway."

Well... I think someone needs some hoarder sensitivity training  :icon_biggrin:

  :laugh: Or not... I'm a hoarder, makes me happy. Happy is good. I've been known to resurrect a few lost causes.

And for anyone who watches the Antique Road Show, you can get lucky...  :icon_biggrin:

ttfn

Title: Re: Identifying Oddball Tube Amp Chassis
Post by: sluckey on June 12, 2023, 05:17:54 pm
Here are a couple lo-rez schematics. The amp is simple enough it should be easy to annotate any unreadable component values. If you search the net for "cordovox schematic" you will find some sites that will sell you the manual. This is a quality power amp worthy of fixing. Add a one or two tube preamp for a nice guitar amp. I'm thinking Ampeg. There's enough juice in that PT to power about 60-70 tubes!
Title: Re: Identifying Oddball Tube Amp Chassis
Post by: mresistor on June 12, 2023, 06:31:43 pm
Yup Cordovox Accordian amp..  the preamp was remote and used those connectors that look sorta like octal sockets but aren't. I have some from organs.


here is a link with pics   https://www.libertybellows.com/shop/c/p/5606---Cordovox-CG-Tube-Amplifier-x63257453.htm
Title: Re: Identifying Oddball Tube Amp Chassis
Post by: AmberB on June 12, 2023, 07:07:09 pm
Those 7591s are good power tubes, generally around 35 watts push pull, although I think they can be pushed a bit more than that.  I've seen them used in hi fi receivers and I think Ampeg had a couple of amps that used them.
Title: Re: Identifying Oddball Tube Amp Chassis
Post by: mresistor on June 13, 2023, 08:41:53 am
In a 7591 the cathode is tied to the suppressor grid, the tube was designed for ultra-linear operation.
Title: Re: Identifying Oddball Tube Amp Chassis
Post by: sluckey on June 13, 2023, 09:08:15 am
Ampeg used the 7591s in about a dozen amps, Geminis, Jets, Mercurys, maybe more. Some were cathode biased, others were fixed bias. The Hammond AO-63 used fixed biased 7195s. I've never seen a U/L 7195 7591 circuit.
Title: Re: Identifying Oddball Tube Amp Chassis
Post by: mresistor on June 13, 2023, 11:14:47 am
I'm just saying that was what they were designed for and not what  they were ultimately used for.


From the valve museum.


"
We have data from Sylvania who describe the 7591 as a power output tube that is capable of 43 Watts of output at very low distortion when operated in push-pull and Class AB1. Or in Ultra linear configuration, an output of 32 Watts with a 400 Volt HT. RCA describe the 7591 as designed for high quality audio amplification.
Title: Re: Identifying Oddball Tube Amp Chassis
Post by: sluckey on June 13, 2023, 11:35:11 am
The fact that I've never seen a U/L 7591 circuit does not mean they don't exist. However, just because Sylvania mentioned Ultra linear configuration does not imply the tubes were designed specifically for U/L. I believe that any tetrode or pentode power tube can be operated in U/L mode. That's a circuit design feature, not a tube design feature.
Title: Re: Identifying Oddball Tube Amp Chassis
Post by: PRR on June 13, 2023, 08:03:04 pm
> not a tube design feature.

A tube selling "feature". Don't wait until they ask, just put it on the sheet!
Title: Re: Identifying Oddball Tube Amp Chassis
Post by: mresistor on June 14, 2023, 08:35:57 am
It was definitely a feature that was intended in the development of the tube as it was used hi-fi equipment where high power and low diistortion is desireable.   Here is some interesting info on the 7591 and the 7868. Sluckey somewhere I read about it but I digress I have no idea where that tidbit is now. Interesting that originally the 7591s were selling for less than $3 a piece.



https://web.archive.org/web/20131122142111/http://www.jumpjet.info/Pioneering-Wireless/eMagazines/VTV/VTV20.pdf (https://web.archive.org/web/20131122142111/http://www.jumpjet.info/Pioneering-Wireless/eMagazines/VTV/VTV20.pdf)