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Princeton-like all octal Guitar Amp from an old PA system
Princeton-like all octal Guitar Amp from an old PA system
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Old 9th June 2021, 11:52 PM   #1
Huckleberry is offline Huckleberry  Canada
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Default Princeton-like all octal Guitar Amp from an old PA system

Hi All,

Part of the reason I wanted to join this forum back in 2010 was the project that I'm about to embark on. I've worked as a teacher for 25 years and all this time there was a dusty old PA amp way up in the top corner shelf of my parts supply room. It turns out it was the heart of the original PA system here at the school back in the late 40s early 50s.

I've been reading up on Octal Amps from this time and realized that 1. I have the tubes (I've tested the amp and it still works and I have many other tubes as well -- looks like there used to be radio courses here back in the 60s) 2. I have lots of extra parts -- and some interested students 3. It looks like an old Princeton Amp with a 5V3, 6SC7, 6SL7, and two 6V6s.

Here are some pics... I'd like to keep this project short and sweet and cheap -- my budget for this year is blown. The circuit board is small and it looks like a lot of the soldering is point to point.

I've looked at Rob Robinette's site which was very helpful but I'd like to hear your opinions on this project. I've already relocated the output transformer to the top side 'cause I think it looks cool. What do you think?

IMG_7871.jpg - Google Drive
IMG_7872.jpg - Google Drive
IMG_7874.jpg - Google Drive
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Old 10th June 2021, 02:52 AM   #2
PRR is offline PRR  United States
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Princeton-like all octal Guitar Amp from an old PA system
No maker or logo?? Possibly "student built" (with faculty oversight). That chassis and cage were standard parts into the 1970s. They *may* even be Hammond.

I wonder if Hammond Mfg would be interested in pictures. Those sure are "Hammond!" irons, and the provenance reflects well on them. (So if you do find a bad iron, you can try for a donation they can take off their taxes.)

The white "Armaco" caps remind me of Japanese production, though I have never seen that brand. Our Canadian members may know if this was a private-brand of one of the radio houses. - - - - hmmmmm, Armaco and Calrad may have been the same marketing company: http://www.audioheritage.org/vbullet...-old-tube-amps

In general you do NOT want to make major changes. The difference between schoolhouse PA and guitar amp is not-much. Frequently we just adapt 1/4 plug to mike jack.
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Old 10th June 2021, 03:43 AM   #3
trobbins is offline trobbins  Australia
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With a blown budget, do you have digital meters or perhaps even test equipment there from bygone days?

Would the school safety code/policy allow anyone to test the amp when energised, or require some form of electrical safety certification to connect a guitar to the amp ?
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Old 10th June 2021, 05:21 AM   #4
Huckleberry is offline Huckleberry  Canada
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Thanks for the quick replies. I agree PRR, I'll keep the changes small mostly 'cause I don't want to screw up my first guitar amp project but also because I think my students are only going to be around for a couple more weeks and then summer holidays!

I'm the Applied Skills Team Leader and the Electronics and Computer Studies teacher. I have scopes, multimeters, freq. generators, you name it. What I don't have are large Filter Caps ... the old ones were bleeding something horrible. I think they were 16uf 450V so I was thinking of sourcing to Digikey or Mouser but they don't seem to have them. Any thoughts?
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Old 10th June 2021, 06:21 AM   #5
jjasniew is offline jjasniew  United States
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Princeton-like all octal Guitar Amp from an old PA system
Ebay for those filter caps. Just search for 450V capacitor...

~30 years ago, I wrote a short article entitled "Converting Integrated/PA Tube Amps into Guitar Amps". You can find its text captured here; Converting Integrated/PA Tube Amps into Guitar Amps.. Looks like people are still doing this :')

I would try for a single point ground. There's many techniques, one that works OK is to isolate the 1/4" input jack ground from chassis and wire the ground side over to wherever the internal wiring ground physically connects to the chassis - hopefully at a single point.

Sometimes on these old amps they just didnt care and made the whole chassis ground, with parts soldered directly to the steel wherever convenient. Sometimes they ran a ground bus above the chassis metal, connecting to it along the way and to chassis at only one point. Depends on the amp.

The idea is to not add a ground loop through the chassis steel, when you mount the 1/4" guitar jack to it. You can get them with isolated mounting, but they're not as strong as the old standard Switchcraft receptacle.

I didnt look at the pictures closely; be sure you use a 3 wire AC cord with its ground wire strongly connected to the chassis steel. Usually the 1st mod I'd make. If you have the time to hack out the cutout, an IEC receptacle would allow you to use any old computer AC cord.

Given your equipment set, looks like you could demo the distortion the amp could make, loading it with resistors, then a speaker padded way down so they could hear (at a low level) what the 'scope is showing for a waveshape. That's certainly one thing you could show your students about this old amp and what it can do for electric guitar.

Good Luck!
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Last edited by jjasniew; 10th June 2021 at 06:26 AM.
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Old 10th June 2021, 07:45 PM   #6
PRR is offline PRR  United States
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Princeton-like all octal Guitar Amp from an old PA system
16 or 20u, same difference. Don't over-do it, but old e-caps were -50%/+100% tolerance so round up.

450V caps are in DigiMouse's catalogs, though may be out of stock.

Guitar amp part specialists:
Hoffman Tube amp parts, Guitar amp parts
Weber Speakers – www.tedweber.com

Of the two, Hoffman has given me very much better service. Boxes go out same-day, best USPS way, though way up the Maine coast is still 4 days away. Yes, the cross-border mail is congested and tax may be confounding. Perhaps the Canadians here will know who is the go-to for shipping to Vancouver.
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Old 10th June 2021, 08:49 PM   #7
Huckleberry is offline Huckleberry  Canada
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Thanks JJasniew! Those are great tips especially getting the multiple chassis grounds sorted. Following PRR's advice I'm re-using the Cap can from Sprague.

Trying to figure out my output transformer wiring just now.

Cheers. J
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Old 10th June 2021, 09:27 PM   #8
Huckleberry is offline Huckleberry  Canada
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Hi PRR. I got 3 X 1 meg pots and 3 20u 450V caps (Vishay) for under $100 shipping incl. (I think I paid too much but wth) Now I'm wrestling with point to point soldering or this old board I scavenged out of a Navy freq. generator. I'm thinking stand-offs and a ground bar... too much? Btw I included a close up of the Cap can and the old board with output trans coming into the chassis.
IMG_7876.jpg - Google Drive
IMG_7877.jpg - Google Drive
IMG_7878.jpg - Google Drive
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Old 11th June 2021, 08:49 PM   #9
Huckleberry is offline Huckleberry  Canada
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Ok update ... I've traced the circuit and it turns out the 6SC7 is not feeding signal into the 6SL7 like I thought, instead its sending its signal to an 6V6. The 6SL7 is connected separately from the SC7 to the microphone and Guitar input. So I'm going to modify this setup to create an all octal 5E3 either with the same tubes or with two SL7s. Btw I've been digging around my supply room and found these. Can I use these as Filter Caps? -- They must be 50 years old by now.
IMG_7885.jpeg - Google Drive
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