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Ampeg SVT Blueline 1969/70

#41
So I thought I might post a short update on progress so far after the amazing help and suggestions I got above.

the amp has been completely stripped and cleaned inside and out. A painstakingly slow process which also helped me learn the whole amp. I manage to save, clean and/or rebuild all the original pots, switches and jacks with the exception of the main volume pot where the wiper had corroded and snapped.

I am now in the process of reassembling the preamp section with all the original screws and mounting plates.

once finished I have found a tech to help me go through the power amp to make sure it’s safe and the transformers still transform! Still a possibility 240v is possible on this one but I need a pro to do that. Also got the cap kit to install from Fliptop.

still a long way to go but I will get it all together before stumping up for new 6550s.

Thanks again to all.
IMG_2770.jpeg
IMG_2771.jpeg
 
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#43
beans-on-toast said:
The white cement resistors are best mounted on their side. When cement resistors blow, flames come out the glue side. If that is facing down, the PCB can be damaged. And always mount these resistors a mm or so above the board, not sitting on it.
Click to expand...


It's fairly common to remove the diodes across the screen resistors as well. Ampeg didn't even place them on many amps.

The diodes are essentially to protect the screen resistors if the output tubes are pushed into extreme overload. However, the resulting current may cause a solder trace on the board to fail. Better to protect the circuit board and let the screen resistor blow IMHO.
 
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#45
Greg Evans said:
Thanks for the pointers both! I’m no Tech but I’m guessing the diodes referred to are D15-20?
Click to expand...

Correct.

Additionally, Ampeg issued a Service Bulletin for modern SVT to up the screen resistors to 220ohm 1/2W flameless. AFAIK this should be a sensible mod for your amp as well.

Resistor Spec Sheet and Service Bulletin attached:
 

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#46
Wasnex said:
It's fairly common to remove the diodes across the screen resistors as well. Ampeg didn't even place them on many amps.

The diodes are essentially to protect the screen resistors if the output tubes are pushed into extreme overload. However, the resulting current may cause a solder trace on the board to fail. Better to protect the circuit board and let the screen resistor blow IMHO.
Click to expand...

Those screen resistor diodes are pretty much useless. Modern schematics show them but have a note DNP, do not place, they are not installed.
 
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#47
Thanks again both. Very useful!!

any thoughts on the Power and Filament Transformers? I haven’t been able to tell if there are 240v wiring options available. It is definitely a mid 71 frame as all caps are May 71 or later.
 
#48
Greg Evans said:
Thanks again both. Very useful!!

any thoughts on the Power and Filament Transformers? I haven’t been able to tell if there are 240v wiring options available. It is definitely a mid 71 frame as all caps are May 71 or later.
Click to expand...

Since the amp was originally designed to run 6550, Heyboer and Mercury Mags have off-the-shelf options. Mercury Mags tends to be a bit more expensive. I believe standard (vintage style) PTs from both companies are dual voltage. Not certain on the filament transformers but you can have one custom wound if necessary.

My 69 came to me with a Mercury Mags 6550 transformer, which is not the correct transformer. I had Heyboer wind a custom 6146 transformer. Can't comment on the windings, but the leads of the Heyboer appear to use a bit smaller gauge wire. The amp sounds great with the new (correct) transformer.

I believe Heyboer makes the transformers offered by Fliptops and Mojotone.

TAD used to carry the PT and Filament transformer for vintage SVTs. Looks like they currently have a dual voltage filament transformer, but there is a note on the PT (Unavailable till further notice).

Might be cheaper to get a custom transformer wound.

If you want to do that:

You probably wont need them, but here are the Heyboer open circuit AC specs
upload_2024-1-3_2-20-43.png


Mercury Mags are slightly different...the screen windings are 2V higher.
upload_2024-1-3_2-21-46.png


As you see, they both have dual 120V primaries, so they can also be set up for 240V.

This is the specs I found.
Later version for 6550s:
PT 8930072:
600v @ 0.75A
330v @ 0.07A
150v @ 0.025A
I believe these are DC voltages at full load

Heater Transformer 8930073:
6.3v @ 12.05A
This is AC

Also found this document and added the note in the call out:
upload_2024-1-3_2-28-26.jpeg


I can't say with certainty that I am correct, but I believe the screen supply typically idles pretty close to 350V DC.
 
#49
Super useful Wasnex! I am still hoping my originals are still viable but need a tech to assess them. I do not understand transformer theory at all!

I have looked at both options you mention. Still hoping my originals can be wired for 240.
Thanks again for the useful info!
 
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#51
Hiya,

yes I saw that post and understand. As I’ve confirmed it’s a mid 71 I was hoping it might be a different transformer.
I have a step down anyway so will keep going saving transformers till last. It’s gonna cost enough just to get it up and running!
 
#54
The help has been amazing! It is my amp and it is progressing well. I have completed the preamp with a full deep clean, recapped, new tube sockets, all jacks and switches cleaned or replaced. Now fully reassembled

I am now doing to same in the Power amp now and just taking my time, learning as I go.

when I get to the mains wiring I will hand it over to an experienced tech to test and/or replace PT/VT and OT and rewire to make safe.
Will post some piccies soon.

Greg
 
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