Bringing back a messed-with small single-ended amp... help!

[brad347]

OK so what I have here is a real simple old tube amp that I got for 50 bucks. I was hoping it would be maybe similar to a tweed champ. Well, I fired it up and it does work, but someone has monkeyed with this thing in a major way. There are globbed up terrible looking solder joints, an amateurish splice from the OT to the non-original "baldwin" speaker (twisted together wires covered in masking tape!) and other such craziness.

The amp looks like it could be pretty cool. 6V6 single ended with a funny preamp tube (6SF5 high-mu triode, octal base). All the tubes are old stock Tung Sol and look great!

I drew out freehand a rough schematic. I want advice on what to do. It does pass signal but is very anemic sounding... low output and all. Here are issues i've noticed.

1. There has been a replacement pot/switch combo put on the tone knob. You actually turn the amp on by turning the tone knob clockwise. The switch works fine but one end of the tone pot isn't hooked up to anything, not even grounded. You can see I left it 'floating' on the schematic because that's how it was left in the amp. I get the distinct feeling this is not supposed to be this way.

2. The resistor that connects from grid to ground of the 6SF5 (two green stripes in the pic) meaasures very high resistance, the 20 megohm range on my meter reads it as infinite. I'm not sure what the two green bands mean, but I'm getting the feeling it's maybe not supposed to be open-circuit (if it is in fact open circuit and not just a really high resistance). How would I know what should go here?

3. the 10k resistor between the two filter caps has drifted 33% high (reads 13.3k).

4. What's up with the .05 uf cap between the 6V6s 200 ohm cathode resistor and ground? Is that supposed to be that way? It looks like it may not be the same brand as the other two .05 caps in the amp.

5. metering across the + and - terminals of the speaker gets a very low resistance (less than half an ohm). What could this mean?

So that's what I've got. This amp looks like it could be cool, can any of the experts tell me anything about it and advise me what I need to do (as a hobbyist amp-tinkerer) to get it back up to where I can hear it how it's supposed to be?

If I don't like it "once i've heard it" so to speak, I'd definitely consider converting it to a 5F1 tweed champ circuit. But I'm wondering if it's not maybe in that ballpark already once it's up and running. The only X-factor is that weird preamp tube.

Here are the images:

First a gut shot:
http://img.photobucket.com/albums/v283/jazz347/IM000066.jpg

Then my amateurish freehand schematic. Pardon the slop:

http://img.photobucket.com/albums/v283/jazz347/cmischemcopy.jpg

Thanks in advance.

[unklmickey]

1. that isn't unusual. sort of a lousy, interactive configuration, but not rare.

2. middle band is brown? 5.1 Megohm.

3. not great, but not a major issue. fix it.........LATER.

4. either you drew that wrong, or this is a HUGE mistake someone made. an absolute must that the cathode have a DC path to ground. i'm surprised you hear anything at the output.

5. if the secondary of the OT is in parallel, it means they used some decent gauge wire on that winding. don't complain.

...I'd definitely consider converting it to a 5F1 tweed champ circuit.

a 5B2 or 5C2 princeton might also be a simple morph for this, but....

i'd fix what you have first. shouldn't take much...........there ain't an awful lot there.

BTW, any idea who the original manufacturer was?


cheers,

unk

[brad347]

Thanks Unk.

I was told that the original manufacturer was CMI, but I don't see anything written anywhere. The speaker is a "baldwin" speaker and I have no idea if it's original. I was assuming it wasn't.

I definitely didn't draw the schematic wrong re: #4. There is definitely a .05 uf cap between the cathode resistor and ground. You can see it in the pic. It's shittily soldered on and shittily grounded to the chassis. The cathode resistor is the gi-normous resistor you see just to the right of the OT. The cap just to the right of that is soldered to it and then to ground.

I will remove it straight-away and report back.

[hasserl]

The input stage (only stage) is grid leak biased, not cathode biased. I agree about that cap between the power tube cathode and ground, how is DC going to flow if it's blocked by that cap? If the speaker alone, not in parralel with the OT secondary, measures only < 1ohm it obviously has a short in the voice coil.

Tough to make it a Champ or Princeton type circuit with only one gain stage. I'd convert the grid leak bias to cathode bias, & give it a proper input. Clean it up and jam.

[unklmickey]

...Tough to make it a Champ or Princeton type circuit with only one gain stage....

sure. that's why i suggested a 5B2 or 5C2. (in both cases, still an octal, but it's a dual triode.


cheers,

unk

[brad347]

refresher course:

How would I convert the 6SF5 to cathode bias again? Sorry it's been awhile since I learned about the different bias types...

[brad347]

OK so i did some 'brushing up' and realised that I need to eliminate the .01 uf cap and 5 M (?) resistor from the input/grid to ground. Then I need to take the 10K resistor off at least one of the inputs. Then I need to insert a resistor and a bypass cap (maybe 25uf @ 25v?) between the cathode of the 6SF5 and ground.

The question is, what size cathode resistor should I use for this particular tube type? Is there something I should measure to find out what a good value should be, is there a common good value, or is it just "trial and error?"

Question number 2:

Say for the sake of argument that after doing this I didn't like what I heard and I did want to convert this to a 5F1 circuit. What's the easiest/best way to get a noval base socket into that octal base hole?

[JohnLochner]

Brad,
Re: question #2, Weber sells an octal-to-noval adapter. Only $5... that's the way I'd go.

[brad347]

that's the kind of answer I was hoping to hear. What two pins does it tie together?

[unklmickey]

hi Brad,

we haven't heard about the progress on the cathode circuit of the 6V6 of this amp.

did removing that cap make it less anemic?


unk

[brad347]

unk:

By "straight away" I guess I meant "when I get around to it."

So I haven't done it yet! (ha!)

Yesterday I had plenty of time but I was actually playing my guitar. Go figure. What a slacker I am...

[brad347]

OK update!

Removed that capacitor. Seems to help it out a bit. Almost sounds like an amp now. Still very low power but a little more juice than it had. A trifle noisy. I wonder if twisting those heater wires and getting them away from the audio path would help that a touch?

I think the speaker is holding this amp back.

Any ideas on cathode resistor value when I convery the 6SF5 to cathode-biased?

[hasserl]

Brad, looking up the data for the 6F5 online the available data sheets show the typical conditions with grid leak bias, not cathode bias. However, the Tung Sol data sheet shows the tube to be very similar to half of a 12AX7. I would experiment with similar resistor and capacitor values you would use with a 12AX7 w/ split cathodes as a good start.

The AX84 Theory of Operation paper from the AX84 project gives a great explanation of how to set the bias for a preamp triode stage. If you don't have that paper I would go out to AX84.com and search for it. It is full of very good info. I could send you a copy ifyou can't fin it. It's in pdf form.

[brad347]

thank you hasserl. I will look for that paper straight away... er... when I feel like it :D

[unklmickey]

...Any ideas on cathode resistor value when I convery the 6SF5 to cathode-biased?

Brad, i don't think there will be much fruit you can shake from that tree.

i suspect you may be giving up a little gain when you go to cathode bias. (and i don't think you have any to give).

anyway, you'll want to select a cathode resistor such that the plate voltage at idle will be a little more than 1/2 the PS voltage
(BTW, what is the PS voltage?)

and you will definitely want to bypass the cathode resistor. (yes, 25 uF should be reasonable.)

since the PS voltage is probably pretty low, i'll guess you should start around 2k and work from there.

the lower the value of the cathode resistor, the lower the plate voltage at idle.


good luck,

unk

[brad347]

PS voltage... i'm not sure I follow you. What do you mean by PS voltage?

[brad347]

OK update again:

I removed the grid-leak bias components (5M resistor and .01uf cap) and cathode-biased the 6SF5 with a 1K resistor and bypassed it with a 25uf cap.

I also removed the 10k resistor from one of the inputs.

NOW. I don't seem to have given up much gain if any but I don't seem to have gained any either. The amp is still very clean and not very loud.

Something very strange:

When my guitar is plugged in the amp is relatively quiet. With nothing plugged in at all it hums like a nest of livid hornets.

AND when my guitar is plugged in and I turn the volume pot ON MY GUITAR, it has that zvex 'crackle okay' kind of crackle that I associate with DC on a pot. WTF? How could this amp be putting DC on the pot of my guitar? (pot does not crackle with other amps).

What am I missing here?

[unklmickey]

PS voltage... i'm not sure I follow you. What do you mean by PS voltage?


B+

or more specifically B+2 (on the second section of the filter cap)


When my guitar is plugged in the amp is relatively quiet. With nothing plugged in at all it hums like a nest of livid hornets.



that should come as no surprise at all.

first, those are non-shorting jacks, so the inputs are acting like antennae when nothing is connected.

second, you no longer have a path to ground for the grid, so the tube is probably going into saturation when nothing is connected, so the hum is also being distorted.

i'd suggest installing shorting jacks increasing the value on the 10k resistors to 47k or 68k. also connect a 1M resistor between the grid and ground.

what is the cathode voltage?

what is the plate voltage?

what is the voltage at each of the filter caps?



unk

[brad347]

I will take those readings at some point when I'm bored and post back.

[vibroverbus]

OK update again:

I removed the grid-leak bias components (5M resistor and .01uf cap) and cathode-biased the 6SF5 with a 1K resistor and bypassed it with a 25uf cap.

I also removed the 10k resistor from one of the inputs.

NOW. I don't seem to have given up much gain if any but I don't seem to have gained any either. The amp is still very clean and not very loud.

Something very strange:

When my guitar is plugged in the amp is relatively quiet. With nothing plugged in at all it hums like a nest of livid hornets.

AND when my guitar is plugged in and I turn the volume pot ON MY GUITAR, it has that zvex 'crackle okay' kind of crackle that I associate with DC on a pot. WTF? How could this amp be putting DC on the pot of my guitar? (pot does not crackle with other amps).

What am I missing here?

can you post a new schematic just to be clear where we are now?

sounds like you don't have a grid leak resistor going from the grid to ground, so grid leak could very likely be putting DC on your guitar (especially if this thing has monster grid leak & that's how it is usually run). i found one 6F5 datasheet that says the grid resistance shouldn't exceed 1M, which is the basic load impedance / grid resistance for guitar amps anyway, so that's a fine place to start.

the basic operating points in the datasheets for that tube seem to be plate supply ~250v, and bias of ~-2v.

what supply voltage do you have on the front of the 270k load resistor, and what bias voltage do you get (from cathode pin to grid pin when on and idling)?

{edit - never mind, unkl made those points & asked those questions while I was not paying attention...}

[unklmickey]

...{edit - never mind, unkl made those points & asked those questions while I was not paying attention...}


if nothing else, you've made me feel more at home.:AOK

half the posts i've made in various forums, have been edited for the same reason:jo