reservoir caps two different size ok?

[Helmholtz]

But then again, caps under high voltage squeeze up physically and change value, right?

No, there is neither evidence nor a physical justification for such "squeeze effect". I explained this a while ago in an other MEF thread. Measured cap value sometimes increases somewhat with a charge of a few volts, but this is mainly an electrochemical effect.

[g1]

No, there is neither evidence nor a physical justification for such "squeeze effect". I explained this a while ago in an other MEF thread. Measured cap value sometimes increases somewhat with a charge of a few volts, but this is mainly an electrochemical effect.

Chuck was joking about that, that's why he put the winking face after the comment.

[mikepukmel]

The 10uf 500v cap came in and I tack soldered it in parallel with the 30uf, and the amp sounds great now. The bass controls on the amp and guitar are responsive. Turn the controls down and the bass goes away gradually. Both guitar and amp bass on 10, its boomy, but not so awful like with the one cap (or before with 2 x 16uf). Have to get a capacitance meter and see what the values are. Never thought this change could have such a huge impact on the amp's tone. Thanks for the help everyone. All I need to do now is get a small eyelet or turret board in that I can wedge between the transformer and main board.

[Chuck H]

I recognized early on in my mod efforts that filter cap value had an affect on tone. This could be due to a few things. How much interaction is in the power supply causing NFB (or PFB in some designs) can be relegated to frequency passed by the filter/decoupling capacitors. On paper it looks like an irrelevant amount. In an amp with gain in the many hundreds or often thousands, it's relevant. The actual affect depends on what nodes are shared and phase particulars. None of which I have a firm grasp on pre calculating (in case anyone wants to pin me as a "guru" for this observation.?.) I only know the phenomenon is real.

[Chuck H]

Additions in bold type:

I recognized early on in my mod efforts that filter cap value had an affect on tone. This could be due to a few things. How much interaction is in the power supply causing NFB (or PFB in some designs) can be relegated to frequency passed (EDIT: or not passed) by the filter/decoupling capacitors. On paper it looks like an irrelevant amount. In an amp with gain in the many hundreds or often thousands, it's relevant. The actual affect depends on what nodes are shared and phase particulars. None of which I have a firm grasp on pre calculating (in case anyone wants to pin me as a "guru" for this observation.?.) I only know the phenomenon is real.

[nsubulysses]

Asking the power supply tone guru CHuck, most amps have lesser and lesser value caps in the power supply as you go down the nodes from plate, screen, V4, V3, V2, V1 for example. I guess it depends what you're going for, but do you think a low value cap in the V1 position would have an affect on feel and response of an amp (V1 more so than any of the other Vs?)? Like how spongey or tight the pick attack is because the V1 node voltage would fluctuate more with a lower value cap? OR would this mainly be because of varying plate and screen voltages.

[Enzo]

Bigger caps hold the voltage tighter.

[Helmholtz]

There are probably several effects involved.

One of them is that different C values change the dynamic response of the supply voltages regarding sag during high current demand. This means e. g. that the attack/release times of screen grid compression are influenced.

[mikepukmel]

Mechanical question for you all. The amp is morphing into something more or less usable. I have two mechanical issues to solve. First is best way to mount the 2 first reservoir caps. I have them tacked in temporarily on two terminal strips as so:


I thought maybe to have a small turret board made that just fits into that spot, and mount the caps on that. But I have seen some old point to point amps that are all terminal strips. Maybe if the connections are neatened up, its OK to leave them on terminal strips?

The other issue is fixing the grounds on the reverb driver. OK, ok you've mentioned that there are tens or hundreds of thousands of these 60's to early 80's amps all made about the same way, but taking up some of the great info on grounding schemes: the cathode resistor/cap ground side should first be routed to the power supply filter cap that feeds that tube/section, then a wire run from there to chassis ground. The chassis grounds should be laid out in order quietest section grounded on one end of the chassis, noisiest on the other. Main idea anyway.

So, I think here, Im going to disconnect enough stuff so I can lift the pot side of the turret board up, install one new turret next to where the big clump of connections used to be, then split up the ground side of the second preamp tube and the reverb driver, since they are fed from opposite sides of the power supply cap chain. Maybe a few other ground wires need to be moved but getting close, amp sounds good, and not so noisy.

[Chuck H]

I'm pretty cavalier about kludge mounting stuff. I can't see the terminal strip you're using in the photo, but I might just zip tie both caps to the terminal strip with the bodies close to the chassis and then goo them. I hate drilling holes and mounting new things in a built amp. All the metal shavings, etc.

As to the grounds... Sounds like you're on a good track. IME a less hummy reverb has more to do with how the jacks are (or aren't) grounded at the tank. But it IS better to keep the recovery tube ground with the preamp grounds and the driver tube and transformer ground with it's associated filter.

[mikepukmel]

Thanks Chuck. I don't feel so bad now, me not wanting to drill and get metal chips all over the place. Tough to get that new turret installed with the board in the amp. Got it in, didn't ruin anything that I can see. Getting ready to rewire everything and discovered I forgot the little nylon spacers between the insulator board (completely unnecessary in this case) and the turret board, which I don't have, so I can't do the rewiring tonight. There HAS to be a couple of nylon washers in this house someplace. Now, what can I take apart

[mikepukmel]

Chuck: re "IME a less hummy reverb has more to do with how the jacks are (or aren't) grounded at the tank" HELP! This is probably a bad part on my amp. I tired to do the grounding like the photos of the originals, but really hard to get a good photo of the inside back panel of one of those old amps. The ground side of the reverb transformer runs to the footswitch jack. The "out" jack and "in" jacks are grounded to the frame, by the body of the rca jack. I checked the tank, and as you guys said, only one side is grounded to the tank body, the tank body isn't connected to both cable grounds.