martin manning wrote: ↑Sat Oct 06, 2018 7:17 pm
I thought about suggesting that you get a complete amp and start poking around in it, but thought better of it since older amps, which are much easier to work on, are getting pricy and hard to find. This Traynor would be an excellent candidate. https://www.thetubestore.com/lib/thetub ... ematic.pdf
If you don’t have one, your first expenditure should be a decent multimeter. You’d need that for a kit build too. Unless you want to spend more I’d recommend a Fluke 115. I just bought one to give to my son, and it seems like a pretty good meter.
Martin beat me to it but then backed down.
I think the best place to start is a Marshall or Fender amp with a printed circuit and modifying it to original style circuit. If you start with a working amp and use that amp as a test mule for any ideas you have you are more likely (I believe) to quickly understand the principles of how amps work. The other advantages are not having to start with the pitfalls of management that come with a brand new kit like placement of transformers or wiring the power supply from scratch. You can jump right to the fun stuff.
If your first try doesn't work the first time.. make sure your bias isn't floating.. don't ask.
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
Get some good pictures of the inside of the chassis and post them. See if the schematic I posted above matches the hardware. It should be close, but it’s often the case that changes were made over time. From the sound, I suspect you will have to rebuild the power supply section. The good news is the transformers are working. In addition to 60-40 Sn-Pb solder, you’ll need some solder wicking braid and or a solder sucking device.
Fine, now the fun part starts. I think you should start to consider not using it as a guitar amp, but rather as a working example to examine in depth.
I never thought about the possibility to start with an old amp, but afterwards this seems a really good idea
pompeiisneaks wrote: ↑Sat Oct 06, 2018 8:43 pm
C'est plutot 'un projet qui vide mes tiroirs' 'clear' en ce cas veux dire 'vider' plutot que 'transparente'
~Phil
Of course! I just thought this could be an advise against any junk drawer project - as in "clear this idea" (from you mind). Well, actually I am considering starting any such project but I am not sure ...
Probably the best way to start debugging is to pull the preamp tubes starting from the one furthest from the power tubes. If the sound continues with all the preamp tubes removed, then you either have a bad power tube or something wrong on the power section. If the sound goes away when you remove one of the (3?) preamp tubes, then you have a better idea of where in the circuit this is happening. Always start with the tubes instead of jumping straight into the guts.
*MetroAmp PPIMV installed.
*Grounded Power Cord.
*Bias Pot.
*NOS period correct Hammond Choke @ 12 henries.
*Filter Caps Dropped to 32uf and 16uf (like old plexi or tweed).
*Phase inverter changed to 1 meg with 10k tail.
*Negative Feedback resistor is 56k.
*Mix Resistors are 270k.
*Bright cap is either 560pf or 715pf I can't read the cap...
*Second Channel can be defeated with a switch under chassis to make the amp single ended (have to plug into channel one). THIS WAS REVERSED DUE TO NEGATIVE FEEDBACK ISSUES (Squealing).
*Volume Pots are Mil Spec PEC pots.
*All changed circuit components are from the ValveStorm store (metro-amp) and are NOS milspec (caps and resistors) except for the Sprague Orange Drop bright cap and bias pot/two caps.
Bryan gives good advice for narrowing down the location of the problem(s). Do any of the controls affect the sound? This is another way to help determine the source.
Looks like the filter caps are recent, but that does not necessarily mean they are all good. 32u is not all that different from the 40u on the schematic. Is the 16u on the last (I.e. preamp) power supply node, “D” on the schematic?
Did you get the list of mods from the previous owner? Again good news there, as it is in good condition and has not been extensively modified.
PPIMV controls can cause oscillation if the leads are not well placed. Do you have any info as to how or when it started making this noise? IOW, was it working with all of these mods in place at some point?
See if you can familiarize yourself with the layout and its relationship to the schematic. Identify the power supply nodes (positive leads on the filters, except for the bias supply which is a negative voltage). Take some time to get familiar with the tube pins for each tube type, of which there are just two. You’ll want to check voltages at these points, and write them down. Some people like to make a table with a row for each tube and a column for each pin, and fill it in right at the start. I’m assuming you can make these measurements safely.
For future reference by others looking for info on this amp, you should post the gut shots here in case your Dropbox goes away at some point.
As for the noise, It started AFTER the single ended switch was removed. Before that is had an issue with squealing.
So, I imagine something went wrong there. The history of the amp is murky because this was a purchase from Reverb.com that never went quite right.
Pics, Will do.
I appreciate all of the help. You guys are the best.
Last edited by horseblanket on Sun Oct 07, 2018 6:31 pm, edited 1 time in total.
OK, I went through and removed preamp tubes one by one and the sound persisted. I think the tubes are fine.
Now knowing the PPIMV may be an issue, I turned it back and forth and the oscillation did lessen and grow depending on the direction I turned the knob. I assume this is due to the amount of power flowing through it?
Honestly, I don't need a master for this amp. As a garage noodle, I got a Fryette Power Station to control my volume.
Maybe removing the after market Master would help?
If you can’t kill all sound with the MV that says the source is in the power amp. It certainly won’t hurt to disconnect the PPIMV, you can always put it back in. You’ll need to locate the original position of and replace R24 and R34, a couple of 68k 1/2W resistors. The value isn’t too critical; twice that value would be fine. The common point for those will be the place where the joined CCW lugs of the MV are connected, which is also the negative bias voltage, and the other ends go to the eyelets where the clockwise lugs of the pots are connected (the 1k5 power tube grid resistors).
That switch that you describe as "single-ended".. single-ended is something else. It's a power amp design.. you just have a switch that goes where the old channel mixers go.. does it cascade.. hmm lets not care. I am going to go look at pics but if the switch is defeated and the mixers replaced but a ground reference is missing along the way..
Ignore me for now. I'm just typing our of my ass. Been terminating cables since I woke up.
(send guns and coffee)
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
'clear' en ce cas veux dire 'vider' plutot que 'transparente'
