1971 Ampeg SVT problem

[boroman]

Allright. Bought a beautiful blueline SVT - I had one before but I missed so sold the SVT-CL to get another one again. It worked great and was maintained in the U.S. - sprague caps, etc. Someone also made mod that I was thinking of back when I had my previous one - to cut signal wire that is going along with 700+VDC in one connector (hum hum hum) and just separate that wires from it. Cool idea.

Anyway- the point is, now, the amp hums.. I'd say it's 80% ground hum 20% 120hz hum or some sort of that. It's nasty and loud, like total bias mismatch or 2 tubes bad.

1/ Checked all the tubes in a tube tester, sockets and seating -- still hum
2/ Changed tubes to another (good) set -- still hum
3/ Removed all tubes from preamp -- still hum
4/ Pic - Fig.1 - removed 12DW7 (splitter?) from power amp -- no hum, can set bias, there is a signal in the speaker cabinet that can be heard like the amp is on and working well = GOOD
5/ Pic - Fig.2 - removed 12DW7 and 2 x 12BH7 from power amp - no hum, can't set bias, no signal to speaker (which should be about right to this case)

So it made me think nothing wrong with the power amp end (6x6550s) along with 12BH7's. This part of the amp is well. I tried to trace bad solder joint, bad ground, something unconnected, everything seems to be solid and fine. Caps are new.


...Any thoughts?









Also, need some help with 120V rewiring to 240V. Ampeg schematic says "FOR 240V OPERATION BLACK AND BLUE WIRES FROM P.T AND F.T. NEED TO BE CONNECTED TO EACH OTHER" - in my case, amp is wired for 120V as I'm using it with external step-down transformer, and yep, blue and black are joined on the same point in a strip. So, does it mean I should leave them joined but unsolder from the strip? (pic below along with schematic).

schem: https://ampeg.com/support/files/Sche...0SCHEMATIC.pdf

[Jon Snell]

Your schematic is a blank page.
If C5/6 is/are leaky the 12BH7 V3 will not allow their cathodes, to bias the KT88s/5881s correctly.
Change R24/5 to 4.7k from 47k. That will improve stability and drive.

[Helmholtz]

[ATTACH=JSON]
Change R24/5 to 4.7k from 47k. That will improve stability and drive.

4.7k seems too low.
R24/R25 are the cathode resistors of the 12BH7 CFs.
With a cathode voltage of -47V I calculate a cathode current of 22mA and a plate dissipation of 6.6W per section exceeding tube limits (20mA/3.5W).
Am I missing something?

[Jon Snell]

4.7k seems too low.
R24/R25 are the cathode resistors of the 12BH7 CFs.
With a cathode voltage of -47V I calculate a cathode current of 22mA and a plate dissipation of 6.6W per section exceeding tube limits (20mA/3.5W).
Am I missing something?

Whoops! ... should have typed 22k.Don't know where 4.7k came from .......
Typo.

[boroman]

Your schematic is a blank page.
If C5/6 is/are leaky the 12BH7 V3 will not allow their cathodes, to bias the KT88s/5881s correctly.
Change R24/5 to 4.7k from 47k. That will improve stability and drive.

I'm not having any problems with bias, also this amp won't run KT88 (spacing issue) and 5881 as well. At first I thought it was reply to another topic but by mistake landed here (?), but since we are here talking about bias, maybe there's a way to make bias potentiometers less sensitive (e.g 5-10 times less). Sometimes only touching the pot it changes by 5mA, and like 1 milimeter to each side is another +/- 20mA... It's easy to decalibrate it during transport vibrations

[Jon Snell]

I'm not having any problems with bias, also this amp won't run KT88 (spacing issue) and 5881 as well. At first I thought it was reply to another topic but by mistake landed here (?), but since we are here talking about bias, maybe there's a way to make bias potentiometers less sensitive (e.g 5-10 times less). Sometimes only touching the pot it changes by 5mA, and like 1 milimeter to each side is another +/- 20mA... It's easy to decalibrate it during transport vibrations

There is. Calculate the values of the resistor chain, adding resistors at either end of the bias pot to equal the original total resistance;
R21 to 27k, R20 to 47k and VR2 to 4k7.
Would give you a finer response. Repeat for the other side.

Or use a 10 turn pot.
Changing the resistors will be far cheaper to do!

[boroman]

There is. Calculate the values of the resistor chain, adding resistors at either end of the bias pot to equal the original total resistance;
R21 to 27k, R20 to 47k and VR2 to 4k7.
Would give you a finer response. Repeat for the other side.

Or use a 10 turn pot.
Changing the resistors will be far cheaper to do!

Thanks. May I try only with one, like R20 to 27k (and sam on the other side) and look what happens? Maybe it's enough then?
(You also said VR2 change from 15k to 4.7k - which is changing the pot... right? Would want to avoid it)

[Jon Snell]

Thanks. May I try only with one, like R20 to 27k (and sam on the other side) and look what happens? Maybe it's enough then?
(You also said VR2 change from 15k to 4.7k - which is changing the pot... right? Would want to avoid it)

If you want a tighter trim with less movement electrivally, meaning a finer adjustment, replace the pot with a lower value and increase the resistors in the chain to acheive a finer feel and less jumpy adjustment.

Otherwise as I stated, fit a multi turn pot of the same value.
There is no other way if you want to keep the same current availability for the grids of the output valves.

[boroman]

If you want a tighter trim with less movement electrivally, meaning a finer adjustment, replace the pot with a lower value and increase the resistors in the chain to acheive a finer feel and less jumpy adjustment.

Otherwise as I stated, fit a multi turn pot of the same value.
There is no other way if you want to keep the same current availability for the grids of the output valves.

That pots would be impossible to find that match the "top plastic pin" adjustment. All those common pots will have bigger shafts that will not fit.
Really anything other like soldering resistor to each pots' leg to make bigger resistance if I'm calculating right, the range will be lower, but sensitive will be lower too, so better for precise and stable adjustment?

[g1]

For the hum, make sure the preamp is getting it's proper grounds. Sometimes the ground wire breaks in the multi-pin connector.
The schematic you linked in the first post shows the 240V wiring.

[Leo_Gnardo]

If you want a tighter trim with less movement electrivally, meaning a finer adjustment, replace the pot with a lower value and increase the resistors in the chain to acheive a finer feel and less jumpy adjustment.

That pots would be impossible to find that match the "top plastic pin" adjustment. All those common pots will have bigger shafts that will not fit.

Consider using 10K linear trim pots Fender used as bias adjust in many amps. Instead of a plastic post with screwdriver slot, you have to poke your screwdriver inside to turn the pot - double advantage because they are less likely to become damaged or accidentally "readjusted". Available from tubesandmore.com.. Woops last minute check shows out of stock there, but I'm sure you can find 'em elsewhere. Have a look anyway:

https://www.tubesandmore.com/product...-trimmer-screw

[Helmholtz]

Wiring shunt resistors (say 4.7k) across the bias pots will have the same effect as lower value pots.
Result is a smaller adjusting range with better resolution.
But it could happen that the limited range no longer allows to dial in the desired bias.
In this case either the upper or the lower series resistor (depending on direction of desired shift of range) needs to be increased by say 10k.

This is assuming that the bias pots are still good.

[boroman]

For the hum, make sure the preamp is getting it's proper grounds. Sometimes the ground wire breaks in the multi-pin connector.
The schematic you linked in the first post shows the 240V wiring.

Cool tip! So, the ONLY ground connection from preamp to power amp chassis is going via molex connector, right?

Helmholtz - that's why I thought! So, do I need to solder them between 1st and 3rd lug of each?
Leo_Gnardo - many thanks for the link. I think I can find those here locally, but I'd try the resistor mod first probably!

[Helmholtz]

Helmholtz So, do I need to solder them between 1st and 3rd lug of each?

Yes, the 4.7 k would go to between the outer pot lugs.

[g1]

So, the ONLY ground connection from preamp to power amp chassis is going via molex connector, right?

Yes, pin 4 of the molex is the only ground connection from pre to pwr amp.