amp out of tube radio

[tsutt]

Fuchsaudio, what kind of circuit, trans formers do you you in a little rig like that?

[Cygnus X1]

Agreed, Andy that radio is BADASS.
I envy your eye for detail.

[FUCHSAUDIO]

Yeah, should be fun when I get the time to finish it up....,

[Structo]

That is a cool looking radio/ amp.

[shoggoth]

I got a lower voltage regulator, so a lot of the hum is gone, and the heptode is better behaved.

a. The power tube breaks up way too early. I need to re-bias it, which means moving diodes around.

b. The power tube socket is crappy and makes lousy contact. Deoxit didn't help, and soldering to it was a pain. Might just replace it entirely.

c. That heptode (unless its the triode, I'll try bypassing the heptode later) does not produce a super clean tone no matter what. It's always a little bit modulated.

d. Still got some hum when the volume and/or gain is cranked. And this is an amp you want to crank, because 1/8w is not a lot of power. At least I can get rid of it up by turning the gain down now.

e. The heater in one of the two pentodes works, doesn't in the other. Even the one with the working filament produces no sound though, so something's wrong with the sockets or the tubes.

[shoggoth]

My hum problems are gone. The amp is dead silent at max volume right now (which isn't very loud with that little power tube anyhow). The problems were:

a. Low voltage supply regulator needed more supply voltage to properly regulate. I also added a 2.7 ohm resistor & 8700uf filter cap after the voltage regulator to further filter anything that made it through.

b. I had connected the high voltage ground and low voltage ground right after the rectifiers. I moved the ground connection to just after the filter caps on both of them.

Now, the heptode stage still distorts when I want it to be clean. It gets progressively cleaner as I bias closer to 0v. I may need to bias it positive (lots of low voltage radio tubes work with positive grid bias). If I had some data sheets with graphs and other useful information I'd be a lot happier. I'm going to have to think about this for a while and come up with an approach for experimenting with the minimum amount of effort.

I also get a weird effect when I play certain power chords on the top strings, one of the notes will get a weird tremolo effect. That ain't cool.

[shoggoth]

I monkeyed around with the screen resistor on the heptode and it's got a lot more headroom now, so it's cleaner. It's biased at -0.7v right now. If I crank the gain, the triode signal is large enough to send it into overdrive, but heptode overdrive is not a glorious tone. No. No it is not.

I've verified one of the gain stages for the dirty channel is functional. I can get some crunch out of it, but it's not a very appealing crunch right now. At least I've got some curves in the data sheets for the RF pentodes.

[txbluesboy]

These are cool little projects. This a picture of one I did. Its basically a tweed Deluxe circuit with all octal tubes. The tone control is connected to the tuning display up front. It's power scaled so you can use it with the built in 4" speaker inside the amp,however when this pic was taken, a friend of mine was using it at a gig so it's plugged into a 2 X 12 cab hidden under the tablecloth. I love seeing the look on people faces when he plugs into the back of this radio and rocks the house.

[Prairie Dawg]

This is an interesting discussion because you always hear people squawking about how their amps are picking up stray radio signals, now they want to turn radios into amps.

[M Fowler]

This is an interesting discussion because you always hear people squawking about how their amps are picking up stray radio signals, now they want to turn radios into amps.

We do this because we don't want to pickup waves we want to make waves

Mark

[Reeltarded]

It is one of the more complicated ways I have seen of getting your song on the radio.

[shoggoth]

It is one of the more complicated ways I have seen of getting your song on the radio.

Only chance I've got. Turns out there's no market for poorly played Black Sabbath covers...

Anyhow, I've mostly figured out my problems with the "clean" channel. The heptode isn't just a heptode - it's a remote cutoff heptode. That's the source of the annoying distortion I couldn't get rid of.

The best solution I could come up with was to use the heptode as the input stage. I'm using the triode as the second stage. The heptode only puts out 2v from an E chord on single coil pickups, so the gain control can knock that right down to the triode's 1v headroom.

It sounds like the heptode wants to be biased at 0v, and from the data sheets the triode has no choice but to be biased at 0v.

When I play chords it seems to get a smidge crunchy, so the clean isn't as clean as it ought to be. But it's getting there...

My once-clean layout has turned to total hash, because I can't get at the under-board wiring without hours of disassembly. There's a certain amount of shielded-wire spaghetti flying around as I reroute signals all over the place. Oh well, as long as it works.

Knowing it's a remote cutoff heptode, I could probably put together a poor man's compressor. But I'm not going to do that - I'd have to repurpose the triode to providing a control voltage to get the compressor going. That triode needs to act as a gain stage, so no compressor here.

A tremolo is a possibility, if I can get the oscillator plate self-oscillating, although I don't have real estate for a rate knob. It'd be a weird tremolo, since it would be before the gain stages. Something to experiment with though, and probably easier to wire up than the compressor.

[shoggoth]

Success! Mostly...

I shorted the heptode's anode to the filament and put 90v across a few weeks ago, so worked stopped for a while. The replacement just showed up. The clean channel got a lot dirtier with the replacement tube, so I had to monkey with the circuit a bit to clean it up. And now it's mostly good. It's cathode biased via a diode, but I'll have to order a Schottky to get the max headroom from it. But for now, good enough.

The high gain channel takes the clean output and pushes it through two small signal pentodes. That's working, and I've got a potentiometer in there to figure out how much gain to drop between pentodes. So I've got it getting crunchy - now I just need to get it voiced right. I'll put up some clips when I figure out how to record on my iPhone (a friend did it for me the last time).

[shoggoth]

That was stupid. Some more parts from Mouser and a replacement pentode showed up, so I did a bit of work on the amp, and then spent a sad hour trying to figure out why it was squealing horribly, clean channel was silent, made horrible noises when I shut it off, etc.

For some reason I had put the dead heptode tube that I had accidentally shorted out back into the amp.

Almost done w/ this now, just have to fiddle with the bias on the triode so the clean channel is stays clean and I'm calling it done.

[shoggoth]

The amp is pretty much voiced now.

My original plan was for much more gain than I've currently got. Sadly, it turns out the heptode was remote-cutoff - that means I've got two tubes with no headroom. The heptode is the input stage, then to the low-gain triode, then for the "higher gain" channel, it gets two pentodes. Then off to power pentode.

It squeals a bit right now due to oscillation issues, I need to tweak the layout (it's currently an absolute disgrace). My gain pot is smack-dab in the middle of the chassis and has no shielding (a choice forced by the length of shaft & the knurling I needed for the radio's knob), so any wires that run near it cause feedback. Hard to keep them away, but I'll work it out.

I've also think I've managed to annihilate a cap somewhere in the circuit through excessive soldering and desoldering, I need to figure out which one. The amp makes a weird squeal when I turn it off now.

The 70 year old speaker farts out on the low notes at higher volumes, possibly because it's just plain old, possibly because I put my thumb through it at one point and repaired it with Elmer's glue and toilet paper, and possibly because there's only so much you can expect from a 5" speaker. I'm going to put an external speaker jack on the radio so I can plug into a decent speaker cab and see how it sounds.

There's also a "clean" channel that really isn't very right now (might be related to the weird shutdown noise that started recently), might work on it, might decide that I've got a certain amount of crunch on both channels.

Anyhow, here's a clip of the "higher gain" channel. I fiddle with the gain and master volume at various points while a buddy of main plays a riff. Recorded on my phone, so sorry for the lousy quality.