Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: firemedic on September 17, 2012, 04:50:34 pm
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I know some of you guys have seen one.
A buddy just got one & is super anxious for me to recap & service it. Good amp? From the schem it looks interesting......
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Nothing? Really?
Apparently the trem doesn't work but he doesn't have the footswitch, which appears to be a stereo jack controlling reverb too.
I don't know enough about oscillator circuits to tell from the schem if that's what he needs.
I still don't have the amp here but that's just because we are both too busy at the moment for the handoff. Will post w/ results.
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I have one...it was a barn find and is rough but has promise once I clean it up. I will probably set up this one for use with my Fender Rhodes but we'll see what it sounds like first. The grille cloth is the same as older Sunn stuff which is not available anymore and cool looking. The speakers were unique alnico speakers that were similar to JBL's from what I have been able to gather. I am missing one speaker in my amp and am not sure if the one that is there is still good or not. The amps were designed by a couple ex Fender engineers and are basically Twin Reverbs using 6550's and higher voltages....I think they are 105 watts and 120 watts depending on the model. The company was in Arizona in the early 70's and didn't last too long...maybe a couple years. They are well built and use good components.
My amp is somewhat incomplete....any chance you could take detailed pics of this amp and share once you get into it? I'm missing parts of the reverb pan I think and maybe some other things. Would be nice to see what a complete one of these is supposed to look like.
One "error" with the design of these amps is that the preamps and phase inverter all share one filter cap....that is one thing that I will be changing on mine...though I will have to get creative as the board is a PCB and not much room to add caps. I will probably utilize the existing hole in the chassis to use a can cap for the power amp and cut traces and reroute wires to supply it. Then I can use some of the other cap locations on the board for the preamps.
Greg
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Sure, I'll post some pics. It's still not here, grownup life is too busy.
Looks like the main cap complex consists of 4 caps in a series/parallel arrangement, quite robust but it sure takes up a lot of space. Also no voltage divider resistors across them if the schem is to be believed.
The owner also says there are "springs sticking out of it" or words to that effect. This is a working amplifier so you may not be missing any parts, it might just look like it.
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The springs sticking out of it brings to mind the reverb on this one....I wasn't sure if that was the way it was supposed to be or not, but from what I could tell it seemed like the dispensed with the casing around the reverb and just had the inner portion with the transducers and springs and mounted it on top of the plate to protect fingers against the circuitry using springs to allow it to float.....seemed rather pointless, but if it works and is the stock way they went about it, I guess my amp isn't all that weird. I had been planning on just installing a Fender tank in the bottom of the chassis with some RCAs going down to it.
There are no divider resistors in there at all....the caps may be sized high enough to where it isn't a concern, but as I said, I plan to mod this one to add filtering for the preamp nodes, and will probably add a can cap in the existing hole for the power amp duties. I will have to cut traces and add some wiring, and still have to look it over and see if my plan is feasable or not.
When you get the amp you will see that they did some interesting things in how they made it, and some strange things also....but they do look like good quality. I would really love whatever pics you can provide once you get the amp.
Greg
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For whatever reason I can't seem to upload my phone pics. My computer is not tube-based so I'm lost.
I finally got the thing. Since my friend played it at full volume already I figured it couldn't hurt to try her out.
Fantastic sounding amp! But something must be wrong with the speaker(s). At high volume, on attack it gets a brief gnarly scratching noise which I guess may be friction rub? When he used an extension cab, there was no noise, so I must be getting warm.
It is very interesting construction. Just as you described.
Anyhoo, let me peruse the thing, try to tighten up the electronics a little & I'll post my findings.
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Sweet! I was hoping you would get it.
You can email me pics and I'd be happy to post them. My email is soundmasterg at yahoo dot com.
I would also love to hear your findings on it. I've read that they are kind of like a Twin Reverb using 6550's...so I'd expect it to sound similar to that description...does that sound about right? Mine isn't in playing condition yet.
I think you are warm on the speakers....I only have one original speaker in mine and I don't know if it works.....it looks like a large alnico kind of along JBL lines.
Greg
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I can't even get the pics from the phone to the computer. Maybe if throw some water on it......
Anyway, the caps are en route via snail mail. In the meantime I have found that somebody placed 1k resistors in series with 2 of the preamp bypass caps. I guess to reduce gain, give some compression, make it sound vintage? The work is clean and it doesn't look like a total hack so they must have had some specific result in mind.
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I haven't taken an in depth look at mine....it was in a barn and needs to be completely cleaned before I really get into it. You'll have to try that with those bypassed to see if you like it with the mod or without.
I hope you can figure out how to get the pics uploaded as I REALLY want to see them. :icon_biggrin: They would really help me with putting mine back together I think....
Greg
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BTW, how did they do the reverb pan on that amp? Mounted on top of the circuitry with standoffs and springs or inside the cab somewhere? Mine is incomplete with that area....
Greg
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Mounted on top with standoffs & springs exposed for all to see. I tried taking pictures with my real camera but the file sizes are too big!
Soundmaster, you're not missing anything from your amp, that's how they built it. Kinda willy-nilly and odd. Since it's not terribly noisy I left well enough alone.
I replaced the filter caps & paralleled 270k 1w resistors in the series cap arrangements.
Replaced all the e'lytics.
Cathode bypass caps are now 10uF instead of 25uF. Removed the series resistors.
Replaced the tremolo photo-resistor. Shrink-wrapped that. The tremolo works now.
Added 6.8k gridstoppers to the power tubes where there were none.
Added a 50k bias pot in series between the 47k bleed resistor and ground. Nominal bias voltage is -67v per schem., I had been getting -63v without the pot, with higher than nominal rail voltage. Which gives me a fairly usable range, -63 to @-80v w/ @550-560vDC on the plates.
I tested the PI coupling caps just to make sure but they weren't leaking any DC. I figured if they are ok then the rest of the coupling caps are probably ok. Sue me, I'm lazy.
Do you have the schematic, BTW? I can try to post that, anyway. It was super helpful to me.
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If you can find the time, you should fix up your AIMS. It is not a TR clone. Similar, but different enough to be worthwhile.
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I do plan to fix my AIMS up...just haven't had time for awhile due to full time work and school, and I have other projects ahead of it too. One of these days I'll find the time.
Thanks for posting the info on the one you fixed! I'd still really love pics if possible? You can email them direct to me if that is easier, and then I could resize them and post them up here for everyone to see. (soundmasterg at yahoo dot com) There isn't really much out there on these amps and every little bit would help others if they are looking. I have some pics that I can post up here later that I found on the web. I looked for days for pics of the chassis and reverb setup and didn't find a thing. I am missing parts on my reverb so that is why I was asking for how they mounted it. I do have the schematic for the VTB120 which I am pretty sure the same as mine....and I can see that it is similar to the TR but not identical. Is that the same schematic that you have?
So 550-560v on the plates....thats a bit higher than the 520v I had heard before. Thats Sunn territory.....which I think makes for a lousy and hard guitar amp, at least with the Sunn preamp and phase inverter. Maybe it is different with the AIMS. I thought about setting one channel on my amp up for guitar and the other for my Fender Rhodes keyboard, as I think that would make a great keyboard amp.
Greg
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About the plate voltage...
The owner bought a "matched pair" of KT88s that were cryogenically treated for an extra premium, from "upscaleaudio.com". He did not consult w/ me but even if he had, they stated the tubes would be matched & he could have just skipped the cryo treatment.
They are the most mismatched tubes I've ever seen, and with the stock bias arrangement one of them was drawing 71mA @ 544v, while the other drew @44mA. I switched sockets to confirm & it's the tube for sure.
So I adapted. The bias pot enabled me to cool off the idle enough so the high-draw tube was safe, but that left the low-draw KT88 idling @ 25mA.
I'm going to put his Sylvania 6550s back in. They're closely matched even now, and the plate voltage should drop to @545v or so with proper biasing.
He wants a clean machine & that's what it will be.
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I'll email when I get the chance, & the if creek don't rise.
The reverb pan is suspended with springs from 4 standoff bolts. There is no tank cover at all, just the barest essential elements of what's needed to make the pan work. IIRC there are a couple of retaining strips on standoffs above & below the pan to keep the springs from overextending & breaking while guys like us manipulate & hack at the PCB with the cover off. The standoffs themselves are mounted on the PCB cover.
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Here's the schematic & PCB layout....
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The file you posted appears to be just a pdf icon image and not a schematic or layout.
I'm posting the schematic I had, plus some pics I had, and in following posts the pics you provided. If you have an actual schematic and layout of the amp then send it to me with what you have and I can try to convert it into something that I can post here.
Greg
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Another pic
Greg
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2nd pic
Greg
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3rd pic.
Greg
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Here are the layout and schematic that firemedic sent me.
Greg
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Thanks Greg, for posting all the stuff I couldn't figure out how to upload. Hopefully some other folks here will find it useful when they find an AIMS out behind some dumpster.
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Schem and PCB in single PDF file.
Maybe for Doug's collection?
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Schem and PCB in single PDF file.
That's funny! I did the very same thing today. :icon_biggrin:
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> I did the very same
I'm sorry... stepped toes?
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I'm sorry... stepped toes?
Not at all. I was thinking more like two old codger's with not much to do today.
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Thanks Greg, for posting all the stuff I couldn't figure out how to upload. Hopefully some other folks here will find it useful when they find an AIMS out behind some dumpster.
Happy to help firemedic! Thanks again for sending them my way...it will help a lto when I get around to messing with the amp.
I sent the schematics and layout file to Doug already. Maybe he wants them in a single pdf...is so its easy enough to do.
Greg