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#1 2013-02-17 09:28:55

simondolanbass
Member
From: Montréal, Québec, Canada
Registered: 2011-11-28
Posts: 14
Website

Ampeg SVT-VR Output transfo primary DC resistance

Hi,

I'm servicing a SVT-VR. I unplugged the primary of the OT and I measured 22.4 ohm and 23.6 ohm on each winding. A total of 50 ohm.

I think that it's very low. What do you think about it?

One tube was bad and some plate resistors and screen resistors are burned.

I ordered new resistors and I hope that the OT is good.

Thanks

SImon

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#2 2013-02-17 18:42:31

hangman
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From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: Ampeg SVT-VR Output transfo primary DC resistance

That doesn't seem too low to me.   The v4 is about double that... But who knows?   I wouldn't worry too much.

If the screen resistors  were 22ohm make sure you change them to 220 ohm.  Loud technologies remade the same mistake SLM made when they released the svt-vr and used 22 ohm screen resistors and left a number of diodes in that had be removed from the design.

Also,  regarding the svt-vr  check the tube retainers.   Are they the right size? All of the early SVT's made overseas used tube retainers that were too small,  where the spring attached to the retainer cap,  the spring would make direct contact with the 6550,  which after many hours of use would etch into the glass and cause the vacuum to become compromised.   I will take some photos and post them later.

It is hard to find the correct tube retainers,   The ones ce distribution/antique electronic supply,   New sensor both carry are the incorrect version.   I have a bunch of the correct ones if you need any.

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#3 2013-02-18 13:46:18

simondolanbass
Member
From: Montréal, Québec, Canada
Registered: 2011-11-28
Posts: 14
Website

Re: Ampeg SVT-VR Output transfo primary DC resistance

Hi,

The screen resistors are 220 ohm 1/2 W. Should I raise to 1 W? or they act like fuse if the tube become bad?

What about the diodes? Should I remove them ? they are parallel to the screen resistors.

The retainers are large enough.

Thanks

SImon

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#4 2013-02-18 21:14:18

hangman
Banned
From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: Ampeg SVT-VR Output transfo primary DC resistance

Ampeg uses 1/2 watt,  and unless there is a catastrophic failure they will be fine. 

If d1,d2,d3,d7,d8,and d9 are still there take them out.   They would be the ones in parallel with the screen resistors.

I would take a close look at the tube retainers to be sure.  They will fit on the 6550,  but if the spring touches it at the top.  It will cause problems

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