Express in single ended format?

[geoffrod]

I was thinking the same thing. A 4K7 or a bad cathode ground or a bad resistor. How about a bad tube?

Also,if the amp does not have an MV, the grid leak resistor could have a shaky ground.

I once removed an MV from a build and forgot to put a grid leak in When I fired it up, the cathode voltage started at 18V and went all the way up to 60V. It rose by about 1 volt per second. The resistor started smoking. A poor ground could mimic this.

edit: Do you have one end of R25 going to ground? I know the schemo shows one end as bias - but that's wrong. Is that why there's 300V on pin 5?

hey Bigbeck,Firestorm,
thanks for the ongoing help.
the cathode resistor is most definitely 470ohm,
one thing i do not have is the grid leak resistor , i will add that this arvo.
another thing i changed last night was i lifted the leg of R21 going to C12
which i supose in effect is taking another grid leak resistor out of the pic,
i will add that back in as well.
I dont have a presence pot in the circuit yet so R24 is just going to ground at the moment.

also i measured something a bit wierd last night, the 1k resistor going to pin 4 via pin6 of the 6v6, there was exactely the same voltage at pin6 as there was at pin4, like the resistor was shorted???
when i started the amp up last night after doing a little bit of lead dressing, i got a woosh sound right up, like it was filling up, once it got to a point it stopped, and the ammp was relatively quiet.
i played my guitar thru it but it was late so no real assesment there yet.
hopefully the grid leak resistor will fix the high cathode voltage.
bigbeck, the 300v to pin 5 was a typo at the time, it should have read pin6.

so the size of the grid leak???? is 220k ok or should it be something else?

cheers
Geoff

[Firestorm]

Are you following the Derailment schematic or the one you originally posted? The original one has some issues: (edit): THREE capacitors labelled C12, "too many" gain stages (though the last one only has a gain of about 2), it's a fixed bias affair, whereas you're using cathode bias ...

You might want to repost your measured voltages without typos; based on the list you first posted, it shows the screen voltage higher than the plate. Is the transformer connected to something other than B+? Are you sure you're using a proper single-ended transformer?

As to the drop between pin 6 and pin 4, there's only going to be a 3 volt difference. Some meters won't see that.

You do need a grid resistor on the 6V6. 220K is just fine. With cathode bias you can go as high as 500K (470K), but you don't need to.

[geoffrod]

Are you following the Derailment schematic or the one you originally posted? The original one has some issues: (edit): THREE capacitors labelled C12, "too many" gain stages (though the last one only has a gain of about 2), it's a fixed bias affair, whereas you're using cathode bias ...

You might want to repost your measured voltages without typos; based on the list you first posted, it shows the screen voltage higher than the plate. Is the transformer connected to something other than B+? Are you sure you're using a proper single-ended transformer?

As to the drop between pin 6 and pin 4, there's only going to be a 3 volt difference. Some meters won't see that.

You do need a grid resistor on the 6V6. 220K is just fine. With cathode bias you can go as high as 500K (470K), but you don't need to.

ok so i have attached a copy of the schematic i drew, bare in mind i dont have the presence in place as yet, and no grid leake resistor on the schamatic.

cheers
geoff

[bigbeck]

Put the 220K in the same position as it shows in the first schematic - one end goes to ground. I'm very suprised the cathode resistor didn't fry and turn to powder. That's what happened to mine. Maybe the tube is shot? I'd get the grid leak in there and if the amp still doesn't work - try another 6V6. Lack of a ground reference on the power tube is SUPPOSED to fry it.

[geoffrod]

Put the 220K in the same position as it shows in the first schematic - one end goes to ground. I'm very suprised the cathode resistor didn't fry and turn to powder. That's what happened to mine. Maybe the tube is shot? I'd get the grid leak in there and if the amp still doesn't work - try another 6V6. Lack of a ground reference on the power is SUPPOSED to fry it.

Well now that you mention it, i did fry a resistor in there earlier, turned it on and saw some pretty blue sparks inside the cement block, but i replaced it and haven't fryed the new one as yet.
hopefully i haven't stuffed the valve

[bigbeck]

You may have also cooked a resistor in the power supply. Check voltage on pins 3&4. Pin 4(screen) should be five or more volts LOWER than pin 3(anode). If the voltages are Ok - try another power tube.

[Firestorm]

On the new schematic: you really don't have the heater connected to pin 8, do you? It's just a drawing "typo," right?

[geoffrod]

On the new schematic: you really don't have the heater connected to pin 8, do you? It's just a drawing "typo," right?

yeah thats a typo mate
i have a heater conection on the cathode with 100r resistors

edit:
ok it is crankin.
cathode voltage is back down to 21v

i only had a 470k resistor to put in the grid stop, with the size make a difference to the tone???

i will check all the voltages now

[geoffrod]

Ok here goes

B+ = 370v
B2 = 368v
B3 = 300v
B4 = 282v

V1b

pin6 = 178v
pin8 = 1.56v

V1a

pin1 = 203v
pin3 = 2.81v

V2a

pin1 = 265v
pin3 = 3.26v

V2b

pin6 = 168v
pin8 = 29v

6v6

pin3 = 369v
pin4 = 363v
pin8 = 21.95v

how do these look

cheers
geoff

[bigbeck]

They look good. What's the tone like?

470K instead of 220K will attenuate the signal a little less.

You didn't cook the power tube?

[geoffrod]

They look good. What's the tone like?

470K instead of 220K will attenuate the signal a little less.

You didn't cook the power tube?

nope didn't cook it
the tone sounds dam good
but i would like it to go from clean to dam mean without gaining as much volume.
so what do i need to do to achieve this.

just on the tone, it can be a little high/harshish, not too harsh, maybe to do with the fact i am using a tele.

i think i am gonna love this amp.
might have to fit a VVR to keep the missus happy

[Firestorm]

i have a heater conection on the cathode with 100r resistors

Clever clever. I've seen that trick done with center-tapped heater windings. Should work fine with 100Rs to kill off the single-ended heater hum.

A couple of things to tweak: try 220K, 200K, 180K, 150K even 100K for the grid resistor. The lower values will kind of smooth things out.

You should measure your AC signal voltage to 6V6 at full tilt to make sure it doesn't peak above 21.95V (or whatever the cathode voltage becomes if you change the grid resistor).

If you can live without the Presence control, you could try feeding the power amp from the cathode of the last triode like on the Derailment schematic. That will drop the signal into the PA by about 3dB and could halp tame things a bit.

Check how much current you have going through the 6V6 at 0 signal to make sure you don't have the 6V6 running at more than 14W. 369VDC plate volts may be highish for a 6V6 in Class A. If it's running hot, you'll burn through tubes pretty quick. If you have lower voltage taps on your PT, you could try those. Or you could use a larger dropping resistor to cut your screen voltage. Or you could feed the screen through a zener diode, maybe 50V/5W (in Class A, the screen current should never change direction so it won't add any switching noise.)

And finally, try rolling a few different triodes in the preamps: 5751, 12AY7, etc.

All good fun.

[bigbeck]

the tone sounds dam good
but i would like it to go from clean to dam mean without gaining as much volume.
so what do i need to do to achieve this.

just on the tone, it can be a little high/harshish, not too harsh, maybe to do with the fact i am using a tele.

i think i am gonna love this amp.
might have to fit a VVR to keep the missus happy

Yes,Tele + Wreck is not the best combination.I know, I have a couple Tele's, a Strat and an Epiphone SG. The SG sounds the best,then the Strat and poor old tele is last. I do have other amps the sound best when my tele is played through them.

You can get nice preamp distortion at lower volumes by installing an MV as in the Derailment schematic..

Firestorm is correct about the highish plate V's. I wouldn't sweat it,unless I was using expensive NOS tubes. Cheap JJ's sound good and can take 400V without breaking a sweat. Besides your true anode voltage is measured anode to cathode which will give you 22V less than the voltage that you posted.

[bigbeck]

If you can't get enough preamp distortion by installing the MV,you can put a 1uf cathode bypass cap on stage 3 or 4. This will give you a lot more preamp distortion. It would be best to put this cap on a switch because the amp will probably sound like sh!t at high volumes.

For using a Tele with my Express, I put a .01 uf cap in parallel with the .002 cap. On a switch,of course. It takes a lot of the ice-pick away. Just remember,the more you deviate from the .002 or .0047 value, the less wreckish it becomes.

[Firestorm]

You could also operate the 6V6 in "fake" triode mode -- add a switch if you want. Triode will have about half the output so you can go "meaner" at lower volume. You'll lose some odd order harmonic distortion (if you like that sort of thing).