Loud hissing from new build/conversion.

[Justin Thomas]

I can follow that...

Justin

http://music-electronics-forum.com/a...2-dscn6420.jpg

[Stratz]

Well, I swapped the plate leads. Still quiet with no NFB in line but as soon as I add any resistance with the decade box it starts to get noisy. Just the opposite of what I was hoping for.....
With the plate leads swapped if I pull V1 there's no effect (exact opposite of before I swapped them). If I pull V2 or the PI it goes dead... no sound.

I still have the micro-phonic tube in V1 issue no matter what tube I put in there as well as the problem with having to turn the guitar's volume up to 10 to get it to sound loud and full. Anything under wide open on the guitar and it sounds quiet and thin.

Time to sleep on this one and hit it again tomorrow.
I thought you had it figured out too....

[Justin Thomas]

Rats. In that case I'd leave the NFB out til the rest of it gets sorted. Next target: parasitic oscillations...

Justin

[Stratz]

Hi all. Just following up on this thread for all of those that helped me.
I had gotten so tired of screwing with this amp that I put it on the shelf for a while and did a couple more PA/Hi-Fi conversions.
Last night I decided to dig into it again and I solved the hissing issue. One of the only components that I didn't change was the input jack and that's exactly where the problem was... Right at the beginning of the signal chain. I feel like a fool.

It's working now but it sounds pretty bad. I'm thinking that my plate voltage of ~ 330V is a bit too low for a pair of 6L6's.
Am I correct in suspecting this? I've been using the AA864 Bassman schematic as a guide and the plate voltage of the Bassman is +420v. The Super Reverb is 460V.

Cheers
Rob.

[The Dude]

What's your bias current? The lower plate voltage shouldn't make a radical difference. If your voltages are low across the board (including bias), you may just have to supply more idle current. That's a good place to start anyway.

[Stratz]

What's your bias current? The lower plate voltage shouldn't make a radical difference. If your voltages are low across the board (including bias), you may just have to supply more idle current. That's a good place to start anyway.

Hi Dude. I just took a measurement with a GZ34 and the bias pot wide open and I got 398V.
I'd really like to get this one working but I've hit a wall. I've done about 15 of these conversions and this is the only one that I couldnt get sounding great.
I may remove the LTP PI and put a cathodyne PI in. This is the only conversion that I've done with the Long Tail Pair and the only one that I couldn't get working.

It's got 3 pre amp/driver tubes so I the cathodyne PI is certainly doable. I changed it from cathode bias to fixed/adjustable grid bias and it works great. I've done the same on most all of my conversions and I really like it.
Dont mind the 450v bias cap. I was out of 100v caps at the time

It's a very cool looking amp. Aluminum chassis and a nice cage top.
The work on the circuit looks a bit sloppy right now because I've torn most of it apart a couple of times while troubleshooting.

The first pic is from when it was stock. Second is as it is now.
Thanks for the reply.
Rob.

[The Dude]

Yes, but what is the actual bias current on the output tubes- regardless of B+. That is what we need to find out.

[Stratz]

Sorry about that.
Biased at 74% PD I have 359Vp/39mA

[The Dude]

Okee dokee. I just wanted to make sure we weren't talking crossover distortion or something of the sort.

[Stratz]

Thanks for the help.

[The Dude]

When you say it now sounds "bad", what exactly do you mean? Can you define "bad"? Maybe that will offer some clues.

[Stratz]

When you say it now sounds "bad", what exactly do you mean? Can you define "bad"? Maybe that will offer some clues.

Hi Dude. I went right to work on the amp after my last post. 3 hours later it was sounding fantastic with only one issue.

I was originally using 3x 12AX7's. I tried AT7's/AU7's in V2 and PI slot but it was still unstable (noisy, light oscillation, etc) So I wound up eliminating V1 completely. I also removed the LTP PI and wired it with a Cathodyne PI like a AA964 Princeton but with adjustable bias.

It worked out extremely well but I could use another 1/2 gain stage to get a bit more volume and some more break up. I also put in a set of NOS JAN Phillips 5881's in place of the JJ 6L6's.

The only issue I'm having is that it has a bit of that EL84 "farting out" sound when it's wide pen. Other than that it sounds like any Blackface Fender.

Any ideas on what could be causing the noise.
Thanks
Rob.

[mikepukmel]

Thanks!!!

Hi Stratz,

I can't thank you enough for posting your troubleshooting sessions, and you you resolved the issue. Ive been talking myself into and out of a scratch simple tube amp build, like a tweed style deluxe or champ for about half a year.

If you don't mind a question, is there any reason that you decided to do a point to point rather than mounting a turret or eyelet board and going that route, or just using what you had on hand? I don't think I'd trust myself to do a point to point the first time around.

[Stratz]

It was a PA amp to guitar amp conversion Mike. The amp was built in the 50's and they were pretty much all point to point back then.

What I did was change what needed to be changed to make it a good guitar amp. It sounds just like a black face Fender now.
If I build a scratch amp I'll use turret strips but in this case P to P was a lot easier.

[Justin Thomas]

As another one who converts lots of organs and PAs, the existing chassis layout with sockets, can caps, etc. is not exactly conducive to putting any lathing besides a few terminalnl strips in. These things were manufactured when PTP was the norm and so the layouts follow that practice. I might occasionally be ablento cram a small filter cap board in a corner on occasion, but usually not...

Justin