Amp gurus, round 2 on my traynor..blocking distortion?

[Kapo_Polenton]

So, working on some plexi mods, here is what I did to this bad boys:

1. Normal input, standard Marshall values are 820 here bypassed with a 330uf. My Traynor came with 1.5k resistors here and on high or volume II input. This means I have a 1.5k bypassed with a 220 uf in this stage.

2. High/ "volume II" input. Another 1.5 k resistor here. I bypassed this one with a .68 uf sozo. Traditionally 2.7 k here in a plexi.

3. Mixer resistors I upped from 270k to 470k with a 470 pf bypass cap on the bright pot side (looks reversed but this is where the wiring runs in the amp)

4. Changed the caps to .022uf to tame the bass.

5. NFR changed to 68k to take some edge off the amp.

6. Big filter caps 40/40 replaced with 50/50 FnT.

First run the amp seemed a tad gainy on 4 but not too bad. I noticed a bit of ghosting starting underneath the notes. Tried one more mod - second stage v2 mod. .68 uf bypass cap. farting sound cutting in and out?? I think maybe the tubes are shot. Throw in some EL34's in place of the 6CA7's that were in there. Check plate voltage (448) and plug guitar back in. VERY crunchy and distorted. Sounded a lot like VH1 but at the far end of things and closer to splatty. Power down and reverse that mod and check for cold solder joints. Everything looks ok. Turn it back on, splatty farty cutting in and out distortion. All gain. Hardly cleans up and huge cut in volume. Am I looking at blocking distortion? Was the transformer on it's way out in the first place and someone dumped this thing on me? So many questions.. maybe someone can see the pic below and give me some ideas. I know there are a bunch of builders or people in the know on this site and I have gotten some helpful advice in the past.

 

[LukeCurd]

looks time to put it on a scope.

 

[sled]

time to put it on a scope.

Yep. Without a scope you're shooting in the dark. That said, one consideration is to lift the Cathode Bypass Cap on all the pre-amp stages, especially the large value electrolytics. Low end gain will decrease. Still, you need a scope.

 

[Kapo_Polenton]

Ahhh crap, I was afraid of that. I obviously don't have access to a scope as I don't work enough with amps to justify getting one. Would be useful though. I was thinking maybe filter caps on the board in the power section but besides that you are right, shot in the dark (Ozzy style)

 

[CECamps]

First thing that strikes me is a 220uf Ck on the input stage. That is an insanely huge value! If you are tweaking this amp to be gainy, I wouldn't exceed 1uf on that first stage bypass cap.

Without seeing a schematic, I really can't get a good idea of the circuit--is this 4 cascading gain stages? What's the configuration of the preamp? Assuming again that this is an amp you are tweaking to be gainy, a few things come to mind as far as things to check:

1) The lead dress looks very conducive to parasitic oscillations if the preamp is being tweaked for high gain. Many times, this will yield very nasty, non-musical distortion sounds that will sound blatty/farty and erratic.

2) Inadequate PSU filtering can lead to ghost notes and generally muddy distortion. A general rule of thumb for higher gain circuits is one decoupled filter node per dual triode envelope.

3) Depending on the level of gain you are trying to achieve, you'll want to cut a considerable amount of bass frequencies up front in the signal path. That 220uf Ck on the first stage is working against you big time in this regard. A .022uf coupling cap will not do a thing to tame that. I'd drop the 1st Ck value to no greater than 1uf and cut some bass with interstage low pass RC filters; again assuming this is intended to be a high gain amp.

These are just some blind suggestions based on assumptions and what I see in the pic. Without a schematic, I have no clue how the circuit is configured.

 

[Kapo_Polenton]

Thanks for your reply! I'm actually not going for high gain, just a good solid plexi I can open up on a hotplate so I was trying to replicate that in the preamp. I also wondered if the decoupling caps could be to blame because I changed the filter caps but not bias supply/decoupling caps. They are 8uf rated for 250VDC and all I have are 8uf rated for 100/or 150 vdc so not sure I want to go there yet. I have read on a few tubeamp help pages that these could lead to motorboating and cut outs but not so sure that would have come on this quick.

I attached the schematic that came with the amp but hard to see clearly when you blow it up. Let me dig and find the right schematic and i'll follow up. This is 3 gain stages, somewhere between a bassman/plexi with Traynor's own spin on the rest.

 

[Kapo_Polenton]

The schem on the last page here: http://www.lynx.bc.ca/~jc/YBA1Schems.pdf seems to be the closest to mine but not a hell of a lot clearer. My pic is almost as good! Hopefully there is something in there that pops out at you but I think you are right about that first cap. "hella big" as they say!

 

[sled]

I don't see Cathode Bypass Caps on the schematic. Does your amp actually have them? If so were these added later?

I see several things I'd fix right off...
- Replace the Power cord, remove the ground cap and install a three prong power cord with Ground.
- Lift the heater center tap ground and add a 100 ohm resistor to ground on each.

 

[Kapo_Polenton]

No those were not there.. the sozos and cathode bypass caps are my doing. I also already installed a three prong power cord with it's own ground to the chassis. The pic is just as a reference should there be any signs or wear etc.. on components.

What will the 100 ohm to ground accomplish? noise or stability?

 

[dawnofdreamx97]

does this amp have a choke? i had a similar issue tracking a ghost and it was related to a choke issue.

Also does this amp have an impedance selector? where is the feedback wire ? 4 ohm 8 ohm or 16 ? try to move it to the 4 ohm jack and see if that fixes the ghosting....

 

[Kapo_Polenton]

No choke on this YBA-1. Some did have them but not mine. Also wired straight out to speaker jack at 8 ohms, no selector or 4/16 taps The mystery continues!

 

[sled]

No those were not there.. the sozos and cathode bypass caps are my doing. I also already installed a three prong power cord with it's own ground to the chassis. The pic is just as a reference should there be any signs or wear etc.. on components.

What will the 100 ohm to ground accomplish? noise or stability?

The heater transformer center tap is a path to ground if the heater ever shorts to the plate, thus possibly taking out the transformer. The 100 ohm resistors do two things, ground reference the heater circuit and act like a fuse in the event of an internal tube short, hopefully sparing the heater transformer.

I'd remove the Cathode bypass caps that you've added.

 

[BC Audio]

Check that each of the cathode R/C networks in the preamp are actually grounded. You'll get splatty cutting out if one is open (not grounded).

 

[Greazygeo]

Thanks for your reply! I'm actually not going for high gain, just a good solid plexi I can open up on a hotplate so I was trying to replicate that in the preamp. I also wondered if the decoupling caps could be to blame because I changed the filter caps but not bias supply/decoupling caps. They are 8uf rated for 250VDC and all I have are 8uf rated for 100/or 150 vdc so not sure I want to go there yet. I have read on a few tubeamp help pages that these could lead to motorboating and cut outs but not so sure that would have come on this quick.

I attached the schematic that came with the amp but hard to see clearly when you blow it up. Let me dig and find the right schematic and i'll follow up. This is 3 gain stages, somewhere between a bassman/plexi with Traynor's own spin on the rest.

That is actually a good amp stock. Put it back to stock and turn it up. They just look funky.

 

[GuitarGoat]

Thanks for your reply! I'm actually not going for high gain, just a good solid plexi I can open up on a hotplate so I was trying to replicate that in the preamp. I also wondered if the decoupling caps could be to blame because I changed the filter caps but not bias supply/decoupling caps. They are 8uf rated for 250VDC and all I have are 8uf rated for 100/or 150 vdc so not sure I want to go there yet. I have read on a few tubeamp help pages that these could lead to motorboating and cut outs but not so sure that would have come on this quick.

I attached the schematic that came with the amp but hard to see clearly when you blow it up. Let me dig and find the right schematic and i'll follow up. This is 3 gain stages, somewhere between a bassman/plexi with Traynor's own spin on the rest.

That is actually a good amp stock. Put it back to stock and turn it up. They just look funky.

I'd do this too. A master volume would be nice obit though. I used to have one with a PPIMV and I still miss it...

 

[Kapo_Polenton]

Ok, back to more or less stock today. (going to leave mixer resistors where they are) If all that pans out, I might change resistors in the PI to more Marshall values or just leave that too and go PPIMV. Hopefully the amp roars back to life.

 

[Kapo_Polenton]

UPDATE!

Back up and rocking. Thanks for the help guys. Turns out those bypass caps were the problem. No more blocking distortion or flubby bass.. if anything, it might be a tad too sharp now with the470k mixer resistors and high treble cap. I think the amp is getting there. Still some things I would like to address though. I need to bring down the fizzies some so I think I am going to copy the marshall phase inverter section of the preamp as well as knock down those first two resistors from 1.5k to 820 or 1k but we'll see. One change at a time. NFR also has to come down a bit closer to 47k I think. Take some of the hard edge out of the sound. At 3 on the volume knob I have all the gain I need on this thing for stacking with OD's. Anyway it is a fun project to work on today... I'll try for some clips later. Loud as this thing is though.