1970s Ampeg SVT refurb

[xtian]

Non-working SVT on the bench. Needs new tube set, new caps, little fixes here and there.

First question: Do you leave the vintage spider, or replace it with a new one?

[Stevem]

I always replace old critters with new ones and any old dryed up beer stains I replace with shinny new ones, I mean heck its a restore right? lol!!
But on the real side, on those amps I replace all the close to the board mounted 5 watt block type power resistors with the black non wire wound type and stand them off the board using short sections of alummium or brass tubing to pass the leads thru.
Some of those boards get pretty toasty when 4 or more of those stock resistors go open!

[xtian]

Excellent advice. About the spiders AND the resistors.

[xtian]

Regarding the plate resistors you mentioned replacing, this is what I would normally use:

http://www.mouser.com/ProductDetail/Vis ... 7IOdDE8%3d

But you said "non wire wound." Can you specify?

[Stevem]

Sorry, I ment non inductive types.
Do you prefere a floor jack, or a engine crane to get the chassis up on a work bench?

[Teleguy61]

Sorry, I ment non inductive types.
Do you prefere a floor jack, or a engine crane to get the chassis up on a work bench?

Exactly.
I think a factory option was 2 300lb roadies to carry the damn thing.

[xtian]

Having trouble finding or determining which resistors at mouser are non-inductive. Can you make a recommendation?


For lifting the chassis, I simply lock my knees, bend at the waist, and lift with my neck until the blood squirts out of my eye holes.

[rock_mumbles]

Here's one ... a non-inductive 5 watt 1k 1% ...

http://www.mouser.com/ProductDetail/Vis ... poXn7jg%3d

$3.38 each!!!

[NickC]

Here's one ... a non-inductive 5 watt 1k 1% ...

http://www.mouser.com/ProductDetail/Vis ... poXn7jg%3d

$3.38 each!!!

Real Time Availability
Stock: 0
On Order: 99 View Delivery Dates
Factory Lead-Time: 22 Weeks

[xtian]

Mouser actually has the ones I need, 10ohms:

http://www.mouser.com/Search/ProductDet ... y71-NS5-10

Don't buy them all.

Anyway, waiting to see if client want to spend seven bills on this repair/refurb.

[xtian]

Great gravy, that's a lot of glass. I just measured 200+ watts of absolutely clean sine output at 1KHz.

Channel 1, V1a is not passing signal. Swapped tubes, no change. I love that Ampeg put removable aluminum plates over the circuit boards for easy access! Those popup tube shields are pretty cool, too.

[xtian]

V1a is stumping me. Schematic attached. I've got 300v on plates, zero on the grid (good) and zero at pin 8 cathode. So there is no current flowing thru the triode.

I have confirmed that there is B+ at the anode and a proper path to ground at the cathode, and swapped in a known good tube. Also, reflowed all the pins.

So, by deductive logic, is the ONLY possible issue that my tube's pins are not connected to the circuit?

[Invertiguy]

Maybe a bad/dirty tube socket?

[fusionbear]

I had the exact problem with a refurb I did. I cleaned and re-tensioned the socket and it cam back to life...

[xtian]

I pulled out the PI, and still hear too much hum at the output. I've got the bias set at 0.070v per side, as per spec, and have set the hum balance.


I replaced the two cap cans (40/40/70) with 40/40/30/30 cans, and paralleled the 30/30 sections to approximate the original 70. If one parallels e-caps without adding balancing resistors, can that be a source of hum in the power rail?