Peavey Roadmaster Super Festival SS replacement pots and stability question

[glebert]

I picked up this amp as an attic find recently. Solid state preamp with British and American sections, and 200W tube power amp. Found that there is a stability issue that seems to be related to bad volume pots. I don't know that I have seen a circuit design like this. It seems like if the pot wiper goes open due to crud or damage or even vibration from 200W of tube output power it loses its ground reference and could lead to the kind of stability problem I am seeing. I'm wondering if there should be some sort of bypass resistors or caps to make sure there is some connection to ground, or some other mod to prevent this kind of issue.

Also, several of the volume pots need to be replaced. Having trouble finding something compatible. The preamp board mounts to the front panel via the pot shafts, so I need something kind of close to the originals with the long threaded bushing part. I would rather not have to replace all the pots. I have a feeling I am going to have to run jumper wires since I doubt I can find ones with this same pin footprint. Total shaft length is 3/4", threaded bit diameter is 3/8", body diameter is 7/8".

I haven't called Peavey and asked if they have replacements, but these were made a long time ago and aren't very common so I would not expect them too. Seems like their tech support is harder to get ahold of these days, too.

Thanks in advance,
Greg


Peavey-roadmaster-SS-11-74-Schematics.pdf

[glebert]

Update: I swapped the worst volume pots with similar tonestack pots. It seems to be much better, perhaps even usable for now.

[Enzo]

Called them for parts on a 47 year old amp, eh? They haven't had those in YEARS.

Your photo shows a pot marked 10k linear. DO ANY of them have a part number on them? Not the 137 number. Part number is of the format 71190053 or similar.

[glebert]

Called them for parts on a 47 year old amp, eh? They haven't had those in YEARS.

Your photo shows a pot marked 10k linear. DO ANY of them have a part number on them? Not the 137 number. Part number is of the format 71190053 or similar.

I actually called them to see if they for a schemo before I found the one above. They have a few different flavors of this series, at least two hybrids and an all tube. Just got someone's voicemail so I didn't bother. As I said, I certainly wouldn't expect them to have these parts.

No part numbers on any of the other similar pots.

Edit: if I have to replace I think these Bournes (P/N: 91A1A-B24-B15L) might work, and the long pins could be bent to the proper location.

https://www.digikey.com/en/products/...4-B15L/1089381

[glebert]

Just an update for anyone else looking to replace pots on this series of amp. I ended up using Bourns PDA241-SRT02-103A2 that I got quite cheap at Arrow.com, and shipping was much more reasonable than Digikey, Mouser, etc. Silly Canucks. I wanted to give myself the option of swapping the guts of the new resistor into the old housings if needed, and the leads on the plastic bodied ones above weren't long enough anyway, and I find it easier to make leads off panel mount lugs. The threaded part of the ferrule on these new ones are just a little shorter than the original, I couldn't get a full turn on the faceplate nut so I ended up taking a step drill to the PCB to let the collar at the base of the ferrule sit down in. I don't know that I would do it that way again, but it is working well. Actually a pretty cool sounding amp (even at lower volumes), and a swiss army knife of different sound options.

[g1]

I got quite cheap at Arrow.com, and shipping was much more reasonable than Digikey, Mouser, etc. Silly Canucks.

As far as I know Arrow is not a Canadian company, did they ship from Canadian warehouse or something? Just curious, I freely acknowledge we're silly.

[glebert]

As far as I know Arrow is not a Canadian company, did they ship from Canadian warehouse or something? Just curious, I freely acknowledge we're silly.

Nope, you're right, I was wrong. It was shipped from Reno. I assumed it was coming from over the border because they added an "import tariff" onto the parts, never seen that from a domestic seller before. Somewhere it must have said CA and I assumed Canada rather than California, which is where Arrow is headquartered.

[g1]

I assumed it was coming from over the border because they added an "import tariff" onto the parts, never seen that from a domestic seller before.

That's interesting. I don't think I've seen that before either. But I do see some Bourns listings there and elsewhere that say 'subject to tariff is shipping to USA'. I assume it is because country of origin is China.

[catalin gramada]

PDA series from Bourns are my favourite pots, also available in imperial and metric bushings. My preference is for HRT series as I found SRT are too much touch sensitive for my taste.