Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Rontone on February 28, 2025, 02:41:47 pm
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Picked up this wee bairn on eBay last year, keep meaning to do a mini project with it
Its 7.5" x 4.25", with a tiny 16 ohm speaker inside, Volume Pot with On/Off switch and would have originally come with a matching mini speaker cab to use as a microphone, + & - terminals on the rear to connect with speaker cable and an input transformer
I found a clip of one being played with guitar at https://youtu.be/gOkvnZDJ5_A (https://youtu.be/gOkvnZDJ5_A)
It uses an EZ80 Rectifier and just a 12AT7 for the SE amp
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A few more gut shots
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Here are some notes on the Transformers
I dont understand how the heater wiring is working either, terminals 4 and 5 on the power TX secondary are going to the bulb lamp for the on/off light, yet the grey wire is running to the sockets for the heaters
See the measurements I made for 10VAC into the primary in the attached picture, the heater windings have over 8V across them in total, grey to black wire, red and black just over 6.3v, I dont quite get it....
The HT seems to be a little over 1:1 ratio so its about 250-260V,
Wondering if its worth converting to solid state rectifier which would put the HT up a lot to around 350v ish, but can knock it back down again, and putting another 12**7 type in front, could squeeze a tone control on there,
I have removed the un-needed voltage selector from the back and could fit a fuse holder [no fuse on this unit!] and a mains power switch on the rear panel, volume on the side and maybe a tone control could fit on the back along with a 1/4" input socket :dontknow:
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The amp sounded good in that video - I wasn't sure what to expect with a single 12AT7 doing all the work. I'd totally go solid state on the rectification and add in another preamp tube as you suggested.
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really want to go all out, SS rectifier then hunt up the schematic here for PRR's 1/3W amp, IIRC it's a pre and PP AX7 so you'd need a small OT
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I was intrigued by the idea of PRR's third watt amp, so I went digging and found this page:
https://el34world.com/Forum/index.php?topic=5039.0
It uses 6AU6 tubes - maybe there is a version out there with 12A*7 tubes that I didn't find?
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... hunt up the schematic here for PRR's 1/3W amp, IIRC it's a pre and PP AX7 so you'd need a small OT
... It uses 6AU6 tubes - maybe there is a version out there with 12A*7 tubes that I didn't find?
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You don't need a special "12A*7 version" schematic.
- If you want a 6AU6 output tube, just use it with the output transformer you already have (18kΩ instead of the ~22kΩ OT indicated).
- Consequence of "wrong load" will be maybe a little less output power... which is exactly what you want anyway.
- If you keep the 12A*7 preamp tube, you would simply look at a Champ or similar amp for "Gain Stage --> Volume (and maybe Tone?) --> Gain Stage --> Power Section"
If PRR were here, he'd tell you there's no "special magic" to the 1/3w amp.
- It's nifty that it uses a pot to lower screen-volts on the output 6AU6 to reduce power output, even though actually using that pot generally makes scratchy-noises.
- IIRC, his point in drawing it up was to say one could make a "pentode Fender Champ (https://el34world.com/charts/Schematics/files/Fender/Fender_champ_5c1.pdf)" out of dumpster-tubes & cheap transformers (including a Reverb transformer for the output) without much thought and have a "fine-working amp."
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I was considering just keeping the output stage as it was, and adding a 12**7 stage in front of it, but then the swap to a bridge rectifier will put B+ at around 350V, so would need to be dropped 100V to keep the output stage to "original" specs and keep the same part values
The extra mA draw from the 12**7 will not be a problem to the HT I imagine, and the EZ80 rectifier has 600mA heaters, so removing that should ease the windings a bit!
Then the pentode option, if I fit a 6AU6 with a 7 pin base, which should just fit onto a B9A chassis hole, then I presume the mA draw is higher from the pentode,
I'm wondering what this mains transformer could be rated for, if it has been made as cheap as possible could it be quite low? I've seen very low mA windings on some transformers, could it handle a pentode on there....
I've been dismantling and cleaning it this afternoon, I will check the HT winding wire gauge with some calipers
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... the swap to a bridge rectifier will put B+ at around 350V, so would need to be dropped 100V to keep the output stage to "original" specs and keep the same part values ...
Never convert a "full-wave rectifier" circuit to a "full-wave bridge rectifier" without changing the power transformer.
- A bridge consists of 4 diodes.
- The 2 bridge diodes between the AC wires and the filter cap + terminal exactly match the configuration of a full-wave rectifier (tube or solid-state).
- You could use a bridge rectifier package to replace the tube, connect the "AC" and "+" pins, and not-connect the "-" pin; now you have a solid-state full-wave (non-bridge) rectifier.
Then the pentode option, if I fit a 6AU6 with a 7 pin base, which should just fit onto a B9A chassis hole, then I presume the mA draw is higher from the pentode,
I'm wondering what this mains transformer could be rated for ...
The 6AU6 won't draw more than your 12AT7 does now. At least, not enough to matter.
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Hi HotBluePlates
The HT has no centre tap, I didn't think at first, only when I came to sketching the schematic out I noticed both anodes, pins 1 and 7 are tied together, the other end of the HT winding to ground, so I presume that was half wave?
If so then I can imagine a full wave would make it sound a little better too
Attached is a .sch file with original EZ80 supply and a proposed Bridge Rectifier
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I'm thinking something along the lines of the attached DIYLC file, its not to scale by any means, just a rough idea,
The preceding stage could simply be a Princeton 5F2 style Vol and Tone preamp, remove the volume pot from before the power amp and put a grid leak resistor there instead
Could I add an artificial CT for the heaters as in the layout?
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Need to post a schematic, not a layout drawing.
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^ .sch file has been added on Reply #9 ^
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Got it stripped and cleaned up a bit, you can see how tight things will be in there, so I think just a single Volume control 5F1 style for the first tube
I made a little backplate to size up some parts, can fit a 20mm fuse holder, on/off switch, a 24mm pot and 1/4" jack, some room inside for a 20mm HT Fuse holder too
There is enough space to fit a KBP package rectifier on the end terminal strip, and modern radial caps at 450V fit in nicely
For example I could point to point wire the 1st Filter Cap positive directly from the B+ tag, with a resistor straight to the Output Transformer to hold the cap in place
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Here is an updated .sch file and jpg of the schematic
If anyone can see any problems with doing it like this, or suggestions on values etc?
I'm unsure of values for V2a Plate Load and Cathode resistor, how best to set it up,
Also grid leak resistor values on the way into V2a and V2b, or would some attenuation be better?
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I'd look at making the AT7 a "parallel single ended" otherwise you'll have too much signal (gain) driving the "PA"