Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: firemedic on December 10, 2010, 03:12:26 pm
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Howdy everyone-
I got a Heathkit IO-17 oscilloscope at a hamfest, which works. It's a pretty lame O-scope but as it is tube-driven I'm going to try making it a tube effect platform. The power supply is strong, there are 6 noval sockets and a 7-pin socket to work with. The B+ to these is appropriate for 12A_7 tubes. It also came with NOS tubes, 1- Sylvania 12AX7, 3 no-name 12AU7s, a 6BQ7, 6BH6 and a 6GH8, so I got my money's worth in parts for sure.
Since my amp is a clean-tone wonder with no overdrive, the first thing I want to make is a tube overdrive. I'd like to do a couple of parallel triode gain stages with roughly unity gain so I can plug it into my input. I have plenty of low-gain tubes to help accomplish this. Any ideas?
Also the PT is a 54-195. I can't find info on this but an initial once-over finds over 900 volts at the first filter cap node! If I can use this PT for an amp I may just poach it for that purpose instead.
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If you look at a tube driven active series effects loop .............. you can use the series for more overdrive if there is no effect plugged in. Just a thought. Not sure exactly what you're looking for?
With respect, Tubenit
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That's an excellent way of looking at it, I hadn't thought of before....
I do have the extra stages available in the scope box. Maybe a stacked pot to mix the effect in w/ dry signal.
Eeeeexcellent.....
But, some advice on controlling the overdrive effect gain level, to make it roughly equal to the dry signal, would be most appreciated. Too much voltage rise wouldn't work too well going to the amp input. So I'm back to square one as far as keeping total effect box gain to a reasonable level. I'm inexperienced with cathode followers, maybe that is a good solution? I think I would still have to put at least a lot of series resistance/ voltage division somewhere in the overdrive effect.
You can see why I still have one tube by my name.
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Here is even a better idea!!!!
Use three 9 pin sockets and do a :
5879 boost
12AU7 FX loop
12AX7 One tube reverb
All of them could be switched in or out! You could wire the boost with a relay to have that feature be footswitchable.
I think this could be a VERY cool idea to build!
With respect, Tubenit
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That is a great layout for an effects loop placement, however my amp does not have an effects loop and I wanted to have a more stand-alone type pre-preamp kind of setup, with signal straight from the guitar, so i could use it with any amp. An EL may be the only way tho...
Either way I do it this schem will be quite useful.
I was also thinking about a 5E9A type tremolo in this thing, one that wiggles the bias of a small tube.
I think I read somewhere in the forum that a 5879 pentode is not the ideal instrument input tube. I am keen to build a TOS though, pretty soon.
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Yeah, duh, this post belongs on the effects board. Sorry folks.
Double duh, the Hoffman tube overdrive pedal project & Matchless hotbox look like variations on the theme. They should get me started. I would like to try blending a dry signal in with the overdrive, by placing the overdrive inside a buffered effects loop internal to the effects shoebox.
BTW I'll also be building a 5F2 Princeton in a Kustom Contender chassis over the next month or so. I'll have to use terminal strips but I ain't scared.
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I think I read somewhere in the forum that a 5879 pentode is not the ideal instrument input tube
Take a look at Geezer's HoSo56 which uses a 5879 as the input (1st gainstage) tube. It is a FANTASTIC sounding amp!
http://www.el34world.com/Forum/index.php?topic=6176.0
With respect, Tubenit
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The IO-17 would be an excellent guitar-amp bench 'scope.
> finds over 900 volts at the first filter cap
The CRT needs 600V-2,000V, true, but at 1mA. This is NOT suitable for a Power Amp. Hang a couple 6V6 on it, biased "hot", and it'll sag-down to 200V, 100V, maybe less.
> triode gain stages with roughly unity gain so I can plug it into my input.
If you want any color at all, you MUST gain-up until the tube plate over-loads. For 300V supply, this is like 60V peak. Strong distortion should be available at 50mV 0.05V input. So you need a gain of 60V/0.02V or over 1,200. You pretty much need two gain stages, hit your 60V wall, then attenuate-down to your ~~0.1V cord level.
OR: work one high gain stage at LOW plate voltage. But this is often less satisfactory.
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I guess that's what I meant by "unity gain"- what would be the favored method of attenuating the high gain signal I want to generate, back down to a level my amp input wants to see. The obvious solution is a big series resistor I suppose, or a voltage divider. But you guys have big bag of tricks so I asked.
I thought it would be a good O-scope too and at $20, and working, I couldn't pass it up. But after messing with it & comparing (unfavorably) to my buddy's old Bell & Howell, I figured I'd spend more time tweaking the scope than the amp I was working on. It has no fine sweep control if that is the term.
I don't intend to make this a power amp, just a platform for tube effects. It must have enough current to run the above mentioned tubes & that's all I need.
Incidentally, the PCB at one of the sockets has the right heater connections for a 5879. Yeah. Four sockets are ready for dual triodes. So I have lots of options.
I want to use the original PCB just to see if I can. I'll certainly have to scratch out some of the tracings to prevent shorts, etc., but there's enough room on both sides of the PCB to mount stuff. It won't look great but nothing I own does. I plan to keep the power supply intact, if it ain't broke why should I mess with it?... I went ahead & formed the caps the other day just to be thorough.
It'll take me a while to process what I have to work with, how I want to wire the selector switch, that sort of thing. I want to convert that little Kustom amp to all tubes first.