Modding an old tube amp???

mikje · 25th May 2009, 01:02 AM

I have a Packard Bell 7TU12 set-up I pulled from an old console. The set-up also has a DPA30-4 tuner/control. I'd like to use just the amp, but I'm not sure how to wire it. I think I can trace the on/off switch if I power it up and run it, but I'd like to find out if anyone can steer me in the right direction. I'll also need to put some inputs on it, and I'm not completely sure how to go about finding them. The power switch and input connectors are on the tuner/control section which is hooked up via a 12 pin connector.
I've searched the web and have come up with nothing yet.
The amp has two EL84 tubes/channel and one 9-pin tube/channel that I can't identify. It has 4008 printed on it and E2, K8, and CO printed on it as well. All the tubes are Raytheon/Packard Bell. Judging by drawings that seem similar to this amp, pin 2 on the 9-pin tube is the input. I don't know that all 9-pin tubes in this configuration are the same though.
I can handle recapping it, but I'm not sure about connecting a power switch (amperage too?), a volume control, or input connectors. If I can figure out the "input tube" I can figure out the input connector and a volume control.
Any help will be greatly appreciated. I need a coach on this one.
Thanks,
Mike

Tom Bavis · 25th May 2009, 01:17 AM

Input tubes are probably 12AX7.

7TU12 isn't listed in the Sams Photofact index, but 7TU11 is - #669-11. Try your library.

planet10 · 25th May 2009, 01:33 AM

Looks like a good candidate for an El Cheapo or Baby Huey or Red Light District variant.

Nice layout, Looks like a honkin' power trafo.

dave

Eli Duttman · 25th May 2009, 08:34 PM

Mike,

As is to be expected, I'm biased and STRONGLY suggest a triode wired "El Cheapo" variant be fitted into that old chassis.

Dave's point about the power trafo's heft is spot on. It's highly likely that the tuner/control chassis got its power from the amp's chassis. OTOH, those O/P trafos seem a tad flimsy. The reduced power O/P of triode mode "finals" makes magnetic headroom available for the low freq. error correction signal loop NFB generates. Reduced power O/P is traded for improved fidelity, particularly at the bottom of the audio band.

Any number of speaker designs will perform well mated to 6 honest WPC.

Can a photo of the underside be provided?

mikje · 25th May 2009, 08:50 PM

Thanks Eli,
I'm going to replace the 4 coupling caps with Audiocap PPT Theta 0.22uF 600V Film/Foil Capacitors
http://www.parts-express.com/pe/psho...27-714&scqty=4
and the one larger orange cap with Jantzen 100uF 400V Crosscap Capacitor
http://www.parts-express.com/pe/psho...27-956&scqty=1

Do those caps look like the right ones to use?

Also, for a main circuit breaker, what amperage should I use?

I'm not familiar with the El Cheapo. Can you give me some info on it: cost, additional parts, etc?

Thanks for your help.

Mike

Eli Duttman · 25th May 2009, 11:10 PM

Mike,

Replacing the coupling caps. is a good idea, as they could be leaky. Replacing ALL electrolytic caps. is mandatory. Pay close attention to the multi-section can part, which is in the PSU. 'Lytics literally dry out over time. Either Panasonic ECQ-P(U) or 716P series Orange Drops are quite suitable in the coupling positions and they are modestly priced. A high quality industrial grade 'lytic, from either Panasonic or Nichicon, is satisfactory for the 100 μF./400 WVDC part.

Jim McShane should have the stuff you need, at a fair price.

You have mail!

The LENGTHY saga of "El Cheapo" can be found here.

What tube type is the rectifier? I suspect a 5U4 is installed.

mikje · 25th May 2009, 11:27 PM

Quote:

Originally posted by Eli Duttman

What tube type is the rectifier? I suspect a 5U4 is installed.

Hey Eli,
It's an ELC322, made by Packard Bell.

planet10 · 26th May 2009, 01:16 AM

Quote:

Originally posted by Eli Duttman
STRONGLY suggest a triode wired "El Cheapo"

Mike,

I note that you have Fonkens. We have an El Cheapo variant (EL84 Class A triode, no global feedback, all Solen PS caps, using an HH Scott LK48 as donor). This 3.2 W wonder happily drives Fonkens -- matter of fact that amp, with our experimental set of FE127eN champhered on all 4 front edges was declared best match yet for a friend's new condo. He has been struggling to get something he could live with with Carina and FE126eN Hornshoppe Horns or Cain & Cain Abbys. We loaned him the El Cheapo, Fonkens, and a set of Fonken167 to see if it could help him with his logjam.

You wouldn't suffer a hardship turning your donor into one of these.

dave

Eli Duttman · 26th May 2009, 01:32 AM

Mike,

ELC322 does not compute.

Please check again.

mikje · 26th May 2009, 01:41 AM

Oops... got it backwards: 322ELC. Maybe that will work.

25th May 2009, 01:33 AM	#3
planet10 frugal-phile(tm) diyAudio Moderator Join Date: Oct 2001 Location: Victoria, BC, NA, Sol III Blog Entries: 5	Looks like a good candidate for an El Cheapo or Baby Huey or Red Light District variant. Nice layout, Looks like a honkin' power trafo. dave __________________ community sites t-linespeakers.org, frugal-horn.com, frugal-phile.com ........ commercial site planet10-HiFi p10-hifi forum here at diyA

25th May 2009, 08:34 PM	#4
Eli Duttman diyAudio Member Join Date: Apr 2004 Location: Plainsboro, NJ	Mike, As is to be expected, I'm biased and STRONGLY suggest a triode wired "El Cheapo" variant be fitted into that old chassis. Dave's point about the power trafo's heft is spot on. It's highly likely that the tuner/control chassis got its power from the amp's chassis. OTOH, those O/P trafos seem a tad flimsy. The reduced power O/P of triode mode "finals" makes magnetic headroom available for the low freq. error correction signal loop NFB generates. Reduced power O/P is traded for improved fidelity, particularly at the bottom of the audio band. Any number of speaker designs will perform well mated to 6 honest WPC. Can a photo of the underside be provided? __________________ Eli D.

26th May 2009, 01:32 AM	#9
Eli Duttman diyAudio Member Join Date: Apr 2004 Location: Plainsboro, NJ	Mike, ELC322 does not compute. Please check again. __________________ Eli D.

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25th May 2009, 01:17 AM	#2
Tom Bavis diyAudio Member Join Date: Jun 2002 Location: Macedon NY	Input tubes are probably 12AX7. 7TU12 isn't listed in the Sams Photofact index, but 7TU11 is - #669-11. Try your library.

25th May 2009, 11:10 PM	#6
Eli Duttman diyAudio Member Join Date: Apr 2004 Location: Plainsboro, NJ	Mike, Replacing the coupling caps. is a good idea, as they could be leaky. Replacing ALL electrolytic caps. is mandatory. Pay close attention to the multi-section can part, which is in the PSU. 'Lytics literally dry out over time. Either Panasonic ECQ-P(U) or 716P series Orange Drops are quite suitable in the coupling positions and they are modestly priced. A high quality industrial grade 'lytic, from either Panasonic or Nichicon, is satisfactory for the 100 μF./400 WVDC part. Jim McShane should have the stuff you need, at a fair price. You have mail! The LENGTHY saga of "El Cheapo" can be found here. What tube type is the rectifier? I suspect a 5U4 is installed. __________________ Eli D.

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