Fixing Ampeg SVT Classic

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M Fowler
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Fixing Ampeg SVT Classic

Post by M Fowler »

What a pig these SVT Classic amps which are built in Vietnam transformers and all.

I've seen this amp before for new tubes 6-6550's.

The amp has loud hum on startup and watched LED's light up green and red but that can happen even when a SVT is working well.
One EH 12au7A was bad.

Tore it down, inspected the 220R screen resistors some may have 22R or 22R/diodes in parallel.
Checked all the other resistors and everything looks clean and factory fresh.

I ordered new filter caps since I have it tore down anyway.
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xtian
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Re: Fixing Ampeg SVT Classic

Post by xtian »

They are back breaking bastards. Is this the one where you can't test it unless you reassemble the preamp and power amp into the chassis?
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tubeswell
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Re: Fixing Ampeg SVT Classic

Post by tubeswell »

Good luck. I have a complete set of SVTCL schematics if you need them. I fixed one up 18 months ago. (Haven't had it back yet - fingers crossed)

When the screen grid resistors go in these, it causes an awful hum and the bias LEDs will never light up properly unless everything is fixed. And make sure the 12AU7 drivers are supplying the right grid voltage to the 6550s before you plug the new 6550s back in. I measured about -70 unloaded and somewhere around -47 loaded IIRC.

You should replace all the screen grid resistors with 0.5W fusible 220R. (Because that way, if they blow up again - which they eventually are bound to do - they won't take anything else with them, and it makes quicker work of fixing them, touch-wood :lol: )

Then make sure you have a matched sextet of 6550s to go in (Those Rg2 blew for a reason). You have to get each side balancing to with 1 or 2mA before the bias comparator LEDs will work properly. (i.e. when you have everything else working properly). These auto-protection circuit in these won't let the amp work unless everything is fixed properly. So if you aren't getting a go light on the control panel, you know there's still work to do (rather than risk disconnecting the protection circuit).

Ideally, to match the new sextet, you need to make a bias probe that can measure plate and screen voltage and tube current simultaneously (and handy if you can measure grid voltage too), so that you can measure each 6550 and work out how to balance them properly. I've attached pictures of one I made which gets extensive use in other amps. (I used 1kV insulated wire for the probe cable leads, and an old Russian 6P3S tube base for the probe plug, which rubber plug cover fits snugly over. I attach the probe chassis to the amp chassis with a dedicated ground lead.)

The 6550s in these run at about 660 plate volts and 350 to 380 screen volts IIRC. They run deep in Class B. Only bias the tubes up to about 20mA each (22mA tops) even if the screens are hovering around 390ish, otherwise there will be trouble. YMMV

One of the main things I hate about the design (apart from the multiple chassis and the damn weightiness of it all) is the use of push-on aluminium spade connector terminals everywhere. These are prone to oxidation build up between the contacts. (Not to mention confusion in hooking everything back up again) This is particularly problematic for the main heater filament lead where these attach to the output tube board (see picture) These particular contacts see over 11A in normal operation, and they can surge quite a lot at startup. Over time, the resistance from the oxidation between the contacts increases, and you can get unwanted high voltage spikes here, which cause arcing and scorching of the leads, plugs and board. But wait, it gets worse! These voltage spikes travel back through the filament transformer, which also supplies the bias supply circuit for the bias comparator window etc. The high voltage spikes get intensified by the filament transformer winding ratios and can actually blow the PS for the bias comparator etc (this happened to the one I had to fix - took me three goes to figure out what was happening). The 'cure' is to cut those pesky spade connectors off from the main heater leads, and solder the leads directly to the output tube board heater terminals. I did this and I haven't had the amp back yet (touch wood). (I figure I only needed to do this with those particular terminals, as they were evidently the ones that were scorched, and they see the highest current of all the heater connections). This means that you can't take the output tube board out without completely desoldering those leads, but I'd rather put up with that that have to have the amp back every so often. You also need to thoroughly clean the scorching off the output tube board. I also had to replace all the caps and diodes in the PS for the bias comparator circuit (as well as put new 6550s in).

And don't get the plate and screen supply wires criss-crossed when you put humpty back together again. You don't want 660V going onto those screens. :oops:
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M Fowler
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Re: Fixing Ampeg SVT Classic

Post by M Fowler »

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M Fowler
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Re: Fixing Ampeg SVT Classic

Post by M Fowler »

some of my photos.
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22780163_10209716201933377_816177506873194013_n.jpg
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Tjaldur
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Re: Fixing Ampeg SVT Classic

Post by Tjaldur »

I have 2 in for service, both dropped on the floor, Power transformer internal short in 1, Heater transformer internal short in the other one. Hard to find spares i Europe, but found both in a www in Belgium.


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martin manning
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Re: Fixing Ampeg SVT Classic

Post by martin manning »

Presence of plate and screen voltages can be checked at the sockets with the power tubes out. SVT-CL already has cathode current sense resistors, so you could get idle current from cathode voltage ...if you could get to it. Too bad there isn't access to pin 8's when the cover plate is on, that would make it easy to check individual tubes and rearrange them to balance the two sides.
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M Fowler
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Re: Fixing Ampeg SVT Classic

Post by M Fowler »

Digitech is down the road from me so received the caps the day after I ordered them.

Peeled off the silicone around the old caps, unsoldered, and replaced with new caps.
22780368_10209726396228228_8556699473026610911_n.jpg
Some leaking of two caps.
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Stevem
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Re: Fixing Ampeg SVT Classic

Post by Stevem »

I had two of those last year that had the two stacked boards arcing to each other, lots O Epoxy repair work on them and added jumper wires!
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M Fowler
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Re: Fixing Ampeg SVT Classic

Post by M Fowler »

The SVT went right off my bench, out the door, off to a gig last Friday night.

He said it was great to have volume back again.

I sure do hate taking these amp chassis, umbilical cord and preamp chassis out of the cabinet and back in again.

Mark
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