Is this worth doing? Read somewhere quite a while back that the PT would have to be replaced. Any information accepted. Pete - Dutchman Ampworks, Llc.
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1970 SVT; convert to 6550 from 6146B
- Thread starter prefsnider
- Start date
The screen supply on the 6146B amps is low for 6550, there are ways it can be derived from the existing PT or the PT can be replaced for a proper wound one.
There's some good info in the thread linked below, starting at page 48 or 49 specifically, but I'm sure some of the same folks will reply to this thread shortly
There's some good info in the thread linked below, starting at page 48 or 49 specifically, but I'm sure some of the same folks will reply to this thread shortly
IMHO please don't do it. The remaining 6146-based amps are becoming more and more rare. Please preserve it.
If you intend to push the amp beyond it's power limits, you might be better off with a 6550-based amp. Why? When a tube amp is pushed beyond its limit, the screen grids in the output tubes dissipate extra power. The 6146 has a less robust screen grid than the 6550. If your goal is to crank the snot out of the amp, I would suggest selling it and getting what you want rather than modding it to run 6550s.
If you need a new set of 6146 tubes there are a couple of sources I can think of.
www.surplussales.com
Several of the bigger MI tube vendors sell 6146 as well. Additionally China currently makes 6146s.
I suggest going with NOS 6146B or 6146W as long as you can find them. My 69 is running a set of the Philips 6146W from RF Parts. I bought a pair of matched trios.
I own both a 69 and an 87. They both sound like SVTs, but not exactly alike. The 87 is closer to what I think most people consider the iconic SVT sound and feel. However, the 69 is bigger, more bloomy, and has more clean headroom. I can't say if this is due to the different output tubes or some other factor. I don't think one sounds better than the other. Rather, I think they both excel in different ways.
What is required to convert the amp to 6550? 6550 runs higher screen voltages than 6146. So in addition to rewiring the tube sockets you need some way to bump the screen voltage up from ~230V to ~350V. A tech with the right skills can do this a few different ways, or you can buy a 6550 spec power transformer (which is arguably the best way but expensive).
The early SVTs required a critical reliability update. The next time your amp is serviced, it should be checked for this update. If the update has not been done, make it happen. The update basically involves cutting three traces on the Power PC circuit card, adding a few jumpers, and adding a resistor and filter capacitor.
This is the way I did it in my amp:
I can provide details on the mod, if they are needed...or you can find one of my older posts where I have already dumped the documents and info.
My 69 SVT did not have the reliability update, which is probably why the Power PC is so beat up. It's obvious my amp has blown the output tubes many times over it's long life. Eventually the power transformer failed. When I bought it, it had a new Mercury Magnetics 6550 spec transformer and a resistor network to drop the voltage to a safe range for 6146. The amp sounded really good but was low on power.
Rather than converting the amp to 6550, I gathered data for several years and eventually had Heyboer wind a proper 6146 spec transformer.
If you intend to push the amp beyond it's power limits, you might be better off with a 6550-based amp. Why? When a tube amp is pushed beyond its limit, the screen grids in the output tubes dissipate extra power. The 6146 has a less robust screen grid than the 6550. If your goal is to crank the snot out of the amp, I would suggest selling it and getting what you want rather than modding it to run 6550s.
If you need a new set of 6146 tubes there are a couple of sources I can think of.
Search Results | Surplus Sales of Nebraska
Several of the bigger MI tube vendors sell 6146 as well. Additionally China currently makes 6146s.
I suggest going with NOS 6146B or 6146W as long as you can find them. My 69 is running a set of the Philips 6146W from RF Parts. I bought a pair of matched trios.
I own both a 69 and an 87. They both sound like SVTs, but not exactly alike. The 87 is closer to what I think most people consider the iconic SVT sound and feel. However, the 69 is bigger, more bloomy, and has more clean headroom. I can't say if this is due to the different output tubes or some other factor. I don't think one sounds better than the other. Rather, I think they both excel in different ways.
What is required to convert the amp to 6550? 6550 runs higher screen voltages than 6146. So in addition to rewiring the tube sockets you need some way to bump the screen voltage up from ~230V to ~350V. A tech with the right skills can do this a few different ways, or you can buy a 6550 spec power transformer (which is arguably the best way but expensive).
The early SVTs required a critical reliability update. The next time your amp is serviced, it should be checked for this update. If the update has not been done, make it happen. The update basically involves cutting three traces on the Power PC circuit card, adding a few jumpers, and adding a resistor and filter capacitor.
This is the way I did it in my amp:
I can provide details on the mod, if they are needed...or you can find one of my older posts where I have already dumped the documents and info.
My 69 SVT did not have the reliability update, which is probably why the Power PC is so beat up. It's obvious my amp has blown the output tubes many times over it's long life. Eventually the power transformer failed. When I bought it, it had a new Mercury Magnetics 6550 spec transformer and a resistor network to drop the voltage to a safe range for 6146. The amp sounded really good but was low on power.
Rather than converting the amp to 6550, I gathered data for several years and eventually had Heyboer wind a proper 6146 spec transformer.
