JTM45 Build

[Reeltarded]

here come the pipers with good advice..

[guitarmike2107]

here come the pipers with good advice..

I struggle to tell when you are been sarcastic sometimes, probably just as well though

[Littlewyan]

Makes sense. I did check all of the grounds with my multimeter and read 0ohms, but that doesn't necessarily mean there is still 0ohms resistance when current is flowing . A bit like jump starting a car, using smaller leads takes longer than it does using larger leads as the smaller leads can't take the current load and so resistance increases in the cable. I'll hopefully have time tonight to try this. The ground connections that is, not jump starting a car .

My Express build did use a similar grounding scheme which worked but there are a few differences. Power valve sockets are on the inside of the chassis for this build which means the ground lug hasn't got as good of a connection to the chassis. In the Express the lock washer between the lug and chassis would 'bite' into the chassis, whereas on this build its just biting into the valve socket. Also the tags on the pots are pushing into the back of the front faceplate which means the pots may not be pushed against the chassis as well as they could be. I remember Glen saying his Express squealed until he tightened the pots up.

It is interesting though how I get low frequency oscillation at turn on but then high frequency parasitic oscillation once the valves have warmed up. Motorboating at startup only with ground issues makes sense as the filter caps are drawing a lot of current when charging (I believe) and once charged up they'll....idle if you will.

Anyway it better be a grounding issue after I've just written this long post

[guitarmike2107]

If you do the changes I suggested , even temporarily, it will eliminate the PI ground loops, if you are concerned about the power valve cathode grounds you could run a wire from the valve socket over the 32uf32uf filter cap ground to eliminate that issue too, probably a good thing to do so you don't have all that current running through the chassis

but even after all that the motor boating may well be due to a bad filter cap or leaky coupling caps....or..... when you getting your scope back?

[Littlewyan]

I tried replacing the PI filter cap last night which didn't help so that can be ruled out at least. I'm sending my scope off for repair today but I may be able to take the amp to my Grandad's house at the weekend and use his scope.

[martin manning]

I like the idea of making sure that the grounds are solid. Resistance is what it is, it's the voltage generated across that resistance that increases with current. The motorboating on startup suggests there is something not right with a filter or the grounding. I don't believe you mentioned that before (?).

[Littlewyan]

No I only noticed it a few times before and thought it was because I had the volume up when I turned it on. After trying to recreate it I found that it happens if the valves are already warmed up. So if you switch it on from cold its unlikely to happen, but if you turn it on first and let it warm up for a minute, turn it off then back on again it will do it.

[Reeltarded]

here come the pipers with good advice..

I struggle to tell when you are been sarcastic sometimes, probably just as well though

Even East Scotland should know better. I am serious when it's sarcastic looking. tsk tsk..

[Littlewyan]

Ok I disconnected the Presence pot from the buss bar and ran a wire from that to the PI cap, no difference. Tried putting a jumper cable from the speaker ground to the mains cap, no difference. A minute ago the motorboating actually sustained itself long enough to make both valves red plate and wouldn't stop! I measured the mains voltage on the rectifier on startup and when you first turn it on it starts rising as normal, then when the motorboating occurs it suddenly drops from 470V down to 340V as if the amp is running full blast. Although I have to say I've never been able to get the amp to sag that much! Its also now doing it with all controls on 0. Still can't get it to occur with the PI valve out.

I've also tried disconnecting the V1 and V2 supply altogether to rule out the filter cap for that and it still had the issue. Looks like I'll have to wait for the scope to see what frequency this motorboating is and where it stems from. Maybe its the reservoir cap, but that doesn't explain by it only occurs with V3 in.

[guitarmike2107]

What does the bias voltage do when it motorboats?

Where is your bias circuit grounded, the black wire in the last picture looks like it is going to the wrong turret, may just be the angle though..

going to have another look

[Littlewyan]

The bias voltage stays steady at -41V. However when the valves red plated it did go down to -45V on the grids but thats it. Bias circuit is grounded on the pots. I thought maybe its motorboating because the bias isn't coming up quick enough but as the bias voltage goes up (well down) the motorboating doesn't change at all.

The wire is going to the right turret its just the angle of the wire.

[Littlewyan]

One thing that springs to mind, can you tighten the clamps around the cap cans too much? I wonder if I tightened the mains cap can too much.

[rp]

One thing that springs to mind, can you tighten the clamps around the cap cans too much? I wonder if I tightened the mains cap can too much.

I'm sure it's not that mystical. Initially it sounded like a layout problem in the tone stack, now it really sounds like a wrong part (resistor) or a miss wire - or both which is trouble shooting hell for sure. For the oscilation that you can't seem to budge one really oddball idea comes to mind, is possible you could have a batch of bad mica caps? I had two bad Cliff jacks (probably bootlegs in retrospect) in a row that took two #$@%-ing days to suss out. Who the heck suspects two bad parts in a row? Hang tough, sucks to be in the weeds.

[Littlewyan]

Well I ruled out anything prior to the tonestack as I disconnected the B+ node for it all. The only other silver mica cap in the circuit is the 47pF across the PI anodes and I tried disconnecting this. I really need to see where the motorboating is coming from and where the oscillation is occurring. I vaguely recall before my scope went bang that I picked up oscillation on the power valve grids. The square wave had jaggered edges all around it.

Edit? What if I measured the anode voltages on the PI as I play. Wouldn't that tell me if one side is working much harder than the other? Maybe the choke is giving me grief.

Edit2: Replaced Choke with 390R resistor, no difference.

Edit3: Replaced Mains side of the cap can temporarily, no difference. Tried grounding the wiper of the treble pot so there is no signal going into the PI but still the motorboating occurred. Interesting.

Edit4: After measuring voltages through the amp to see what changes when the motorboating stops it appears to stop as soon as the bias voltage stops going negative at -41V at the junction where the two 220K Grid Leaks meet. However the bias voltage on the power valves goes even further negative until the motorboating stops. V4 has -50V on the grid and V5 has -45V on the grid. Replaced bias capacitors, no difference

[Littlewyan]

Ok found an issue. The PI valve I was using has pins 3 and 4 strapped together internally -_-.

Right, the motorboating has stopped and the issue with the parasitic oscillation/blocking distortion has gotten worse, probably because the signal has become even stronger now that the PI is working correctly. Which means a lot of the troubleshooting I've already done has gone to waste as some of it just involved me listening to see if the motorboating stops. Dam.