Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: judge74 on February 16, 2023, 03:49:48 pm
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I picked up an old Westinghouse suitcase record player a few weeks back. Its turntable doesn’t turn and the amp didn’t make any sound.
I have pulled the amplifier portion out and looked it over I am thinking of turning it into a guitar amp. But which one? It is a stereo amplifier with two 12ax7s two el84 and a 6CA4 rectifier. Considering everything the power transformer is enormous and the output transformer is tiny. I actually don’t quite understand what is going on in the output because there are actually two output transformers and one of them can be switched in and out of the circuit. (I will attach the schematic to a reply)
Anyway I was thinking of building a matchless hotbox style preamp into I am not sure what on the power amp side. If somebody has any other ideas I am all ears. I am looking for something that can get me into chimey slightly overdriven British amp sounds.
Attaching some pics of the guts to the next post too. I did plug it in and test voltages, the unloaded B+ is 356 volts. I am kind of hoping that the 30/30/40/40 can cap that is there is going to be OK and I can just get away with using it.
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Here are some photos and the schematic of what is there now.
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Vox AC15? 1x 12AX7 driver and 2x EL84 and similar voltages, adding the Matchless Hotbox with 2x 12AX7 in front adds one more valve than the stock Westinghouse, will the transformer take the extra heaters...
plenty of good spare wire there too
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Very odd output stage.
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Very odd output stage.
I am thinking that the smaller OT switches (almost) completely out of the circuit with the mono stereo switch. So when that happens both el84’s are using one OT and are parallel class A? Just what I am guessing….
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I picked up an old Westinghouse suitcase record player...
And it does not have any model number??
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Gibson GA-8T/GA-18T Crestline-era: https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga_18t.pdf (https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga_18t.pdf) .
It's basically an EL-84 Princeton. 2 6EU7's (12AX7's with a different pinout), 2 EL-84s, and a 6CA4. I've owned a couple and they sound great, just adjust the coupling cap values to something more sane unless you like TREBLE with a capital T - Leo's values on the Princeton are a good starting point, adjust to taste.
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I picked up an old Westinghouse suitcase record player...
And it does not have any model number??
It is the H-68ACS1. I already found the schematic and attached it.
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With the switch open, would that coil its running through act as a choke also?
The main output TX secondary is connected back to the driver cathodes, is that some kind of 'stereo feedback'?
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Vox AC15? 1x 12AX7 driver and 2x EL84 and similar voltages, adding the Matchless Hotbox with 2x 12AX7 in front adds one more valve than the stock Westinghouse, will the transformer take the extra heaters...
plenty of good spare wire there too
The OT is much smaller than a AC 15 I am scared if I built an AC15 output stage in it I would release the magic smoke..
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There's 2 EL84s on it right now, if that is the right schematic, although the way its wired may mean the OT is strange, if its to set up for this stereo and mono functions
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I found this comment in a different forum where they were talking about a tube stereo that is wired similarly on the OT
"This is like some Grundig amplifiers, where they separately amplify the
sum and the difference signals. That other transformer you see in the
B+ lead extracts the difference signal and then adds it to one side of
the secondary ( L+R ) + (L-R) => 2L, while subtracting it from the
other half of the secondary: ( L + R ) - ( L - R ) => 2L. Clever.
I wonder if they licensed thepatent or what? You might try looking
up the patents at ww.uspto.gov and see if anything interesting pops up."
https://maker.pro/forums/threads/a-single-output-transformerstereo-tube-amp.68830/
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There's 2 EL84s on it right now, if that is the right schematic, although the way its wired may mean the OT is strange, if its to set up for this stereo and mono functions
I am wondering if it needs both of the little output transformers to actually handle 15 watts...
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The Matchless Spitfire 15 looks like it would work with what’s there. I just have to figure out about the OT
https://schematicheaven.net/newamps/matchless_spitfire.pdf
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Yeah, unhook that OT to measure, find out what its all about, is that secondary going to leave you with some strange impedances,
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Ok I unhooked the main OT. I measured resistances and fed some voltage into the primary to see what came out of the secondary.
The primary is center tapped and reads 410 ohms across the whole thing. The secondary is also center tapped and reads 1.1 ohms across.
I used a variac and fed 53.6 volts across the primary and got 1.902 volts from the secondary.
I need to look up again how to do the math to figure out the impedance.
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http://mh-audio.nl/Calculators/OTLoadResistance.html here's the lazy calculator I have been using,
Seems from looking at Matchless amps they use pretty chunky OT's, but have lower impedances on their EL84 amps,
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That works out to 6352 with a 8 ohm speaker. I think that will work because when I looked up replacement ac15 transformers they were running around 6k. Also the ac15 transformers I was looking at were a little bigger but not that much so I think it’s worth a try…
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I have a pair of OTs here from tiny Bell & Howell amps that are really dense and heavy, they are only 2-1/2" x 2" x 1-1/4" in size, had a pair of UL84s on them doing 15w ish into an 8ohm speaker, could be run at 6k6 ohm into a 16ohm speaker for a pair of EL84s
I noticed Peavey Classic 30's have a very tiny output transformer with those 4xEL84s
https://www.ampbooks.com/mobile/classic-circuits/vox-ac15/
This site, at the bottom has some good info and circuit analysis on the AC15 power section
I only just noticed its a copper chassis? It look like metallic enamel paint at first...
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I have a pair of OTs here from tiny Bell & Howell amps that are really dense and heavy, they are only 2-1/2" x 2" x 1-1/4" in size, had a pair of UL84s on them doing 15w ish into an 8ohm speaker, could be run at 6k6 ohm into a 16ohm speaker for a pair of EL84s
I noticed Peavey Classic 30's have a very tiny output transformer with those 4xEL84s
https://www.ampbooks.com/mobile/classic-circuits/vox-ac15/
This site, at the bottom has some good info and circuit analysis on the AC15 power section
I only just noticed its a copper chassis? It look like metallic enamel paint at first...
Thanks, the more I look at 15w transformers the more certain I am this will work. It is actually a steel chassis that’s painted as far as I can tell. I am thinking I will pull off the smaller second transformer and put a little choke I have in my junk box in its place. The matchless has a choke and so do AC15s. I have no clue how to measure a choke though…
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Having a look at some other Westinghouse players and consoles there are copper chassis models, it looks like yours has one of the corner tabs on the inside soldered together, nice, don't use any abrasive cleaning it, keep it shiny with some metal polish
Although it will be soft when pressing down with drillbits, or punching out sockets, I put a bend in an FX pedal case recently not holding it tight enough, drillbit stuck and twisted the box a little bit...
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Here's another project you may be interested in...
http://sluckeyamps.com/dual_lite/dual_lite.htm
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Those chassis are usually copper clad steel. Copper for conductivity and rust prevention and steel for strength. A lot of consumer electronics (tv, radio, console stereos, record players, etc.) from the '60s used the copper clad steel.
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Your Dual Lite project is what brought me to this forum, it is on my list of 'British' things to build, I've got a real fvox tail on a keyring somewhere I can hang on it when its stuffed too!
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It's a small simple amp but it's one of my most played favorites. Lotta Brit tone options.
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"This is like some Grundig amplifiers, where they separately amplify the
sum and the difference signals. That other transformer you see in the
B+ lead extracts the difference signal and then adds it to one side of
the secondary ( L+R ) + (L-R) => 2L, while subtracting it from the
other half of the secondary: ( L + R ) - ( L - R ) => 2L. Clever.
He meant to say, "while subtracting it from the other half of the secondary: ( L + R ) - ( L - R ) => 2R."
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Those chassis are usually copper clad steel. Copper for conductivity and rust prevention and steel for strength. A lot of consumer electronics (tv, radio, console stereos, record players, etc.) from the '60s used the copper clad steel.
Looked a little closer that’s exactly what it is…
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Here's another project you may be interested in...
http://sluckeyamps.com/dual_lite/dual_lite.htm
That looks really nice! Do you have a schematic? Is the Vox side a EF86? I have tons of old tubes but I don’t think I have one of those. Also that is one more tube than I have on my chassis now. I assume the PT would handle it but I don’t have a lot of real estate.
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Schematic is at the bottom of the page.
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https://el34world.com/Forum/index.php?topic=29296.msg326057#msg326057 (https://el34world.com/Forum/index.php?topic=29296.msg326057#msg326057)
Just one other option
to the tube you have, to build something like in the link you must add an ef86 tube
I think It will be interesting to maintain the output with the record player transformer configuration
Franco