regency mr-33 monitoradio to guitar amp conversion

[duffy878]

So I picked up this little shortwave radio at an antique store for 20 bucks with the intention of turning it into a guitar amp. and i did, its now a 3 watt guitar head.

Im posting here because I'm looking for suggestions for improvements to the circuit.

I've attached the original service manual, what the circuit looks like now, and a few shots of the little guy.

thanks in advance!
James

[duffy878]

I'm also considering additions to the circuit like using one of the 9-pin sockets from one of the radio tubes to build in a cascaded preamp or a tremelo circuit.

two of the three pots are currently not being used.

I'm just looking for any opinions ...or feedback from anyone whos done something similiar.

[RWood]

Looks like a great circuit to experiment with. Have you tried bypass capacitors on the 12AX7s? A .68 or 1uF cap in parallel with each 560 ohm resistors might be interesting. You could make them switchable for 'overdrive' mode.

What kind of voltages are you getting on the plates of the preamp tube? I was thinking you might lower the plate load resistors to 100k


RWood

[duffy878]

I'm reading anywhere between 104VDC and 111VDC. lowering the res values to 100k would increase the gain, correct?

As for the bypass caps, it sounds like a good idea to me. Having the caps in would make the tube break up earlier, right? I should be able to do that with a DPST...

This isn't my first build but I am still kind of new to this, so I really appreciate the help RWood.

Another question, when I turn the thing up relatively high and strum hard I get a pretty substantial loss in volume... would that be old tubes, specifically the rect tube, causing that?

[duffy878]

I was doing some poking around today and it appears as though I'm running the power tube much hotter than I had initially thought...

Im getting 198VDC off the trans, rather than 180 VDC.
236 VDC after the 6X4 recto tube
232 VDC at the 6AK6 plate
235 VDC at the 6AK6 grid
and a steady 90 VDC at the grids of the 12AX7 (lower than I initially thought)

these values are significantly higher than those listed in the typical operation section of this data sheet:

http://tubedata.milbert.com/sheets/049/6/6AK6.pdf

but they are still under the maximum ratings.

Do I have anything to worry about? could this be creating too much draw on either the transformers or the rect tube, and in turn creating the volume loss I mentioned in my last post?

any help would be greatly appreciated

[RWood]

You want the screen voltage on the power tube (pin 6) slightly lower than the plate, so increasing the 270 R after the first filter cap will do that. Might try a 1K there instead. That will in turn lower your preamp plates farther, so you might try reducing the 33k to 22k or so to bring them back up.

Is the second filter cap really .25uF? You might go 8uF there and 8uF or 4.7uF on the third one.

Is the volume drop that you described momentary, as in, you hit a chord and it drops, but as the chord is decaying it rises back up? That could be a weak 6X4, as you thought, or not a lot of PT current available. If this persists with a fresh 6X4 then you might try a pair of 1n4007 diodes instead of the tube.

Btw, if you put the bypass caps on switches you'll want to parallel a 100k resistor across the switch contacts - it'll keep them from *popping* when you use the switch. Also, you only need SPST switches, but use what you have.

[duffy878]

The volume drop is momentary then comes back as the chord is decaying. I ordered new tubes the other day, they should be here next week.

I'll have to order some 100k, 1k-1w, and 22k-1w resistors when I get a chance.

For the cathode caps, your saying to wire each cap to each cathode then tie them together to one side of a SPST switch, whereas the other side of the switch goes to ground? I just want to clarify... Also, From what I've read, those caps would really just be changing the frequency response of that section, how exactly would it be an overdrive? And do they have to be electrolytics, or would any 1uF-25V+ cap do the job?

I'm also getting some slight hum, 60hz and 120hz, from my preamp section (figured it out by pulling the preamp tube). It is related to volume. I noticed that my heater lines are actually in series and not parallel, so for example, pins 4 and 5 of the 12ax7 are connected to the heater line and it continues to one pin on the 6 ak6, but pin 9 is just connected to ground. would this account for the noise?

There was a cap from the plate of the 6AK6 to ground (a 0.01 I think) that eliminated the higher hum, would that be something I should reinstall?

Sorry I have so many questions, I'm kind of a sponge when it comes to this stuff. I really appreciate the help RWood.

[RWood]

For the cathode caps, your saying to wire each cap to each cathode then tie them together to one side of a SPST switch, whereas the other side of the switch goes to ground?

Yes, if you want to switch both in and out simultaneously (otherwise use separate switches). Then the 100k resistor is connected to both lugs of the switch(es).

I really can't say how it works, just that it does, especially on the lower powered amps. I had great results on a Silvertone 1471 where I did this, putting the switch on a push-pull tone pot, so it even looked stock.
You could use any 1uF 25 V cap, but people generally use electrolytics. You can also try .68, 4.7, 10, 25uF etc to compare the sounds.

There's hum and there's buzz. Hum can be from filter caps that are leaking, or from bad lead dress on the filament wires. On your series filaments, try to make sure that the filament wires cross any grid wires at a right angle, or better yet drop the grid wires in from above and keep the filaments tight to the chassis, out of the way.

Does this unit have a filament transformer or is it coming straight from line voltage? If it is the later, there is going to be a higher noise floor - it just comes with the territory.

Buzz indicates grounding issues, so you have to condsider if you have multiple ground points, ground loops, etc. There is a boatload of stuff on the internet about how to tackle ground issues, and Randall Aiken and the GeoFx articles are great places to start.

Not sure about the cap from plate to ground, but I have seen that on some 50's amps. Try it and see, although I wouldn't expect much from it.

Don't apologize for asking questions - its a great way to learn!

RWood

[duffy878]

Well I got my new tubes yesterday and put them in. Sure enough that volume loss is still there, which is probably too much current draw on the PT (correct?) So from here I guess I should try SS rectification, that should make it easier for the PT to handle the load seeing as how they are discrete, right?

As for the buzz, I suspect it's the volume pot. its one of those click on ones with the fuse soldered to the back and one of the AC lines. I already know it's coming from the pre-amp because when I pull the tube it's gone. and it's not the jack because if I ground it, the buzz is still there.

I suspect both could also be attributed to some sort of oscillation. Should I try throwing in a grid stopper resistor between the tubes, or replacing the grid stopper at the input of the first stage to see if that solves anything? maybe replace the resistor to ground at the input jack too... let me know what you think.

I'll mess around more with the ground scheme after I get the thing sounding how I want it.

I'm going to wait to order the other parts until I can figure out the volume drop problem. It would be pointless to continue if I can't crank the thing...

[duffy878]

okay, so I replaced the coupling caps and that quieted the background noise down quite a bit. I also changed the 220K plate resistors to 100K and this thing is now sounding much better.

I'm still having the volume drop problem at high volume. Would a SS recto lighten the load on the transformer, possibly fixing this problem?

I am still trying to figure out how I'm going to put in the switch for the bypass caps.

[duffy878]

I just put in the cathode bypass cap switch with 10ufs. those were the only caps that I had laying around. It sounds really nice, I really like the breakup.

I have one more pot I can use. I'm considering putting in a one-knob tone control from here: Adam's Amplifiers: Tone Stacks

The garnet looks the most intriguing to me, has anyone tried this configuration?

I could also use some form of this tone control: http://www.muzique.com/lab/tone3.htm

any thoughts?