Express in single ended format?

[geoffrod]

Ah, I didn't look at that one.

That schematic doesn't show the power supply at all so you need to use a bias circuit just like a regular Express and then and inject it in via the 220K resistor R25 to get the negative voltage on the power tube grid.

got ya
thanks paulster

just on these jones amps versions of SEWreck, they show Master Volume, Is there a reason, considering the originnal Wreck doesn't have it????

[geoffrod]

So, does the method of biasing make a difference to how to power section will behave?
I mean will cathode biased SE power section sound the same as a fixed bias SE power section?
and will it not allow the power supply to sag? if it sags at all(i dont know)
i understand this to be an integral part of the express amps characteristics, which allows it to stay at similar volumes clean to mean.
I guess what i am asking, is it going to be possible for me to achieve this sag, and is the method of biasing the power section going to play any role at all?

cheers
Geoff

[Cliff Schecht]

To keep it short and simple, cathode biased amps tend to sag whereas fixed bias amps have a very quick transient response. This has to do with the fact that cathode bias amps have to fight against the RC time constant of the cathode resistor and bypass capacitor whereas fixed bias amps have no RC time constant and can react much quicker. TBH, I like to implement both in my SE builds so that I can tune in the perfect dynamic response without having to use feedback. Sometimes I'll even make the cathode switchable between and RC bias and ground bias. Of course you have to make sure the grid has a negative bias if you remove the RC network in the cathode or you will quickly blow up your power tube.

[KellyBass]

I love where this thread is going!

Especially since (I confess) i've built an amp around this schematic and got stumped at the power supply.

I just wasn't too sure how I wanted to handle the fixed/cathode bias question...as well as the master volume I'm (and geoffrod) is seeing in the Jones SEwreck. I hate master volumes I'm gonna try it without like in the SExpress schematic.

I sort of like this adjustable cathode bias setup:

http://www.ax84.com/static/corepoweramp ... ematic.pdf

[bigbeck]

Audio clip of Jones Derailment with parallel 6V6's and Edcor XSE15 OT and MPS 285 PT - 360V on plates.

[geoffrod]

Audio clip of Jones Derailment with parallel 6V6's and Edcor XSE15 OT and MPS 285 PT - 360V on plates.

Hey BigBeck,
Yeah i have listened to your clips done by Kyongsteadt, and they sound great, So yyor build is a derailment with parallel 6v6's, do you have the PI in the circuit like dartanion, or straight up as the schemmatic shows? also do yyou have the master Volume?

cheers
Geoff

[bigbeck]

No PI. Yes on the MV, although it's not really needed as the amp sounds best with the MV and gain turned almost all the way up to 3:00.

I also used a modified James/Bax TB stack. I didn't like the one that's on the schematic.

If I were to build another,I think I'd use the tone stack from the current Express schematic. If you don't want/like a mid control,just use a 15K resistor to ground in place of the mid pot. I did that on my Express and am very happy with it.

I used a 5K 15 watt Edcor OT. Also 500K gain pot and a .022 coupling cap on the first stage going into the tone stack.

The B+ voltages are 365 - 360 - 350 - 322 - 320. The dropping resistor on the first filter node is 100R 2 watt. That's pretty much the only changes I made.

[bigbeck]

Hey BigBeck,
Yeah i have listened to your clips done by Kyongsteadt, and they sound great,

cheers
Geoff

Thank you. I like the way Kyoung records his clips. He does good work. However, the amp sounds even better in a live setting. It's loud enough to play with acoustic drums - as long as you don't have a wild drummer.

[KellyBass]

Thanks for the voltages, BigBeck! That'll come in handy...

That's funny, I was just thinking about a bax tone stack for this build. I love them in bass amps. Glad to hear you had good luck with it.

[geoffrod]

Hi all,
I have built my sewreckish amp, but i have a terrible buzz with nothing plugged into the input, plugging a guitar in reduces it somewhat, but the whole chassis is microphonic, tapping on the chassis with my chopsticks can get the ossilation going as well, the whole chassis being microphonic makes every noise get amplified thru to the speaker even with the volume turned down to off.

I have some wierd voltages as well,
B+= 337V
B2= 330V
B3= 279V
B4= 254V

V1b =
Pin6= 156V
pin8= 1.48V
V1a
pin1 = 182v
pin3 = 1.9v
V2a =
pin1 = 237v
pin3 = 2.95v
V2b =
pin6 = 156v
pin8 = 25v

6v6

pin3 = 279v
pin4 = 315v
pin5 = 330v
pin8 = 57v

If anyone could have a look at these voltages and give me some comments, it would be greatly appreciated.
the starge voltages to me are the differences in pin 1 and pin 6 on V1, they come off the same node, thru exactly the same size 100k resistors yet one has a voltage of 156v and the other 182v???
and also the bigger difference between the v2 pin 1 and 6
and also the cathode voltage on v2b, 25V????

then the power tube, cathode voltage doesn't seem right at 57v????

i am not sure if there may be problems with my coupling caps leaking maybe???

cheers
geoff

Edit;
ok so i just played my guitar thru it and apart from the buzz, it seems as though the caps are draining, and i loose sound and volume.

[dave g]

To keep it short and simple, cathode biased amps tend to sag whereas fixed bias amps have a very quick transient response. This has to do with the fact that cathode bias amps have to fight against the RC time constant of the cathode resistor and bypass capacitor whereas fixed bias amps have no RC time constant and can react much quicker.

Not quite. It actually has to do with degenerative feedback through the cathode resistor. The larger the bypass cap, the less feedback there is. Once you get above about 1000uF, I doubt anybody could tell the difference between fixed and cathode biased.

[dartanion]

There is no sag in SE operation. As well, fixed bias works with SE amps. Give it a shot sometime.

[Cliff Schecht]

To keep it short and simple, cathode biased amps tend to sag whereas fixed bias amps have a very quick transient response. This has to do with the fact that cathode bias amps have to fight against the RC time constant of the cathode resistor and bypass capacitor whereas fixed bias amps have no RC time constant and can react much quicker.

Not quite. It actually has to do with degenerative feedback through the cathode resistor. The larger the bypass cap, the less feedback there is. Once you get above about 1000uF, I doubt anybody could tell the difference between fixed and cathode biased.

Makes sense, I was obviously looking at it the wrong way. Thanks for the correction .

[geoffrod]

Hi all,
I have built my sewreckish amp, but i have a terrible buzz with nothing plugged into the input, plugging a guitar in reduces it somewhat, but the whole chassis is microphonic, tapping on the chassis with my chopsticks can get the ossilation going as well, the whole chassis being microphonic makes every noise get amplified thru to the speaker even with the volume turned down to off.

I have some wierd voltages as well,
B+= 337V
B2= 330V
B3= 279V
B4= 254V

V1b =
Pin6= 156V
pin8= 1.48V
V1a
pin1 = 182v
pin3 = 1.9v
V2a =
pin1 = 237v
pin3 = 2.95v
V2b =
pin6 = 156v
pin8 = 25v

6v6

pin3 = 279v
pin4 = 315v
pin5 = 330v
pin8 = 57v

If anyone could have a look at these voltages and give me some comments, it would be greatly appreciated.
the starge voltages to me are the differences in pin 1 and pin 6 on V1, they come off the same node, thru exactly the same size 100k resistors yet one has a voltage of 156v and the other 182v???
and also the bigger difference between the v2 pin 1 and 6
and also the cathode voltage on v2b, 25V????

then the power tube, cathode voltage doesn't seem right at 57v????

i am not sure if there may be problems with my coupling caps leaking maybe???

cheers
geoff

Edit;
ok so i just played my guitar thru it and apart from the buzz, it seems as though the caps are draining, and i loose sound and volume.

Ok so i solved my noisy buzz with nothing plugged in, put a switched jack in the input.
still a bit noisy when i plug into it though, (something to work on).
Here is the wierd thing though. I play my guitar thru itwhen i first turn it on, seems ok until i dig in or try to sustain a note, and the sound bleeds away like the caps are draining when you turn the amp off,then the amp goes dead quiet, that quiet you think it has blown a fuse or something, until you dig in again, i get sound, but this time it bleeds off quicker.

To me it sounds like my caps are draining far to quickly and the power supply cant keep up, What could be the cause of this????
it also seems like if i leave it for long enough the caps fill back up and the noisyness comes back.

i have gotten rid on the microphonic chassis, replaced the first ecc83.

Anyone got any suggestions.
they would be greatly appreciated.

cheers
Geoff

[bigbeck]

I'm assuming you followed the Jones Derailment schematic? No way you should have high voltage on pin 5 of the 6v6,or is that a typo? Check your wiring.

A bad ground at the cathode will give you a high cathode voltage.

You should have different voltages at pin 1 & 6 because the cathode resistors are different values. You'll even get a 10 volt or so different reading if they both had the same value cathode resistors. This is due to tube manufacturing tolerances .

Here's another clip of what it will sound like after you fix it