What to do with Peavey VTB300 (200 watt tube head) with bad PT

[glebert]

I don't like halfwave rectification in power supply (power as opposed to e.g. bias supply).
Reasons:
- Halved ripple frequency and increased ripple voltage, requiring considerably larger filter capacitance,
- Increased rectifier peak currents and increased filter cap ripple current,
- Most of all, it means an average DC current flowing in the PT secondary, magnetizing the PT core and increasing PT losses and buzz.

Good points.

[nevetslab]

I see I posted my possible source of power xfmrs from decommissioned Blackstar 200's onto the wrong thread. But, you did find it on the other one and contacted me about it. So, I just have to clear space on the bench and pull one or both of the amp cabinets up to extract the chassis' and then unmount the power xfmr(s) to see if the Great Spirit has bequeathed a potential power xfmr to solve the dead xfmr of your Peavey VTB300 amp. Got your PM, so will advise with what I find in the next few days.

[glebert]

I see I posted my possible source of power xfmrs from decommissioned Blackstar 200's onto the wrong thread. But, you did find it on the other one and contacted me about it. So, I just have to clear space on the bench and pull one or both of the amp cabinets up to extract the chassis' and then unmount the power xfmr(s) to see if the Great Spirit has bequeathed a potential power xfmr to solve the dead xfmr of your Peavey VTB300 amp. Got your PM, so will advise with what I find in the next few days.

Even if this doesn't work out, I seriously appreciate the offer. Most people wouldn't go to this trouble to help someone they don't really know. This is what makes MEF great!

[kg]

- Halved ripple frequency and increased ripple voltage, requiring considerably larger filter capacitance,

note, this does not necessarily have to be the case. there are two doubler topologies, one of which (the less commonly used) does not suffer the half-wave rectifier refresh frequency:



of course you'll need to double the values of c1 and c2 as they are in series.

peak winding/rectifier currents are higher, as you stated.

however it also relaxes your third objection, that of average DC current in the secondary.

[glebert]

Might as well give an update on what I have found out regarding repairing or replacing this. Heyboer does not do rewinds or repairs. They would prefer to just clone it and make a new one (and potentially add to their design list). They do have a design that might work, for a Hiwatt DR201. Different scheme in that it floats the high current plate winding above the mid-voltage winding. It would probably be about $250 shipped.

Also talked to Mercury Magnetics, just for kicks. The guy there was really nice, and based on my description of the problem wanted to run it passed the tech guy. I think they do some repair work, but haven't heard back. Their DR201 model is over $500.

DR_200wPS.pdf

[drewl]

What about an Ampeg SVT trans?

There's plenty of room on that chassis for a different type.

[Helmholtz]

note, this does not necessarily have to be the case. there are two doubler topologies, one of which (the less commonly used) does not suffer the half-wave rectifier refresh frequency:

I wasn't speaking about doublers but about half wave rectification.
The doubler circuit you're showing rectifies the full wave (i.e. both half waves).

[glebert]

What about an Ampeg SVT trans?

There's plenty of room on that chassis for a different type.

Don't they usually have a separate filament transformer?

[nevetslab]

The SVT HT Power Xfmr only has the Plate supply and screen supply for the KT88/6550 Power Tubes. The Heater xfmr has that winding, along with that for the Bias Supply and the Bipolar Supply to run the IC's. So, not a suitable option.

I've been busy with getting thru a room full of amps that came in from Cochella, along with some other GK gear that tied me up prior to that.

I still have to extract the Power Xfmr(s) from those Blackstar 200S amp chassis to see if either chassis have a working power xfmr. Sorry for the delay on that offer. Should get to that task this weekend. Hopefully one of those power xfmrs will be what you need.

[glebert]

I still have to extract the Power Xfmr(s) from those Blackstar 200S amp chassis to see if either chassis have a working power xfmr. Sorry for the delay on that offer. Should get to that task this weekend. Hopefully one of those power xfmrs will be what you need.

There is absolutely no rush, I've set this aside for now, but still really appreciate the offer!

[nevetslab]

I finally removed the two sets of power and output xfmrs from these Blackstar 200S Amp chassis. The other chassis which didn't appear in the photos had a large PCB sitting above the power tube section, and both it's power and output xfmrs had terminated leads, so I took a look at the power xfmr of that one, powered up. I did pull the end bells off of the other one, which had it's primary and secondary leads cut back some, plus there was a BLK and WHT wire on what I assumed to be the Primary side, cut from the breakout of the coil form.

After revising a power outlet box of mine with a mains fuse ahead of the power switch, letting me have protection during powering up off of the variac/power analyzer, I found the HT supply of the other power xfmr to be 524VAC C/T. 524VAC would yield 740VDC less rectifier drop, and this is no load. So, I'd assume these RED-BLK-RED leads to be the HT supply. Then, there's a pair of BLK-BLK leads which is only 26VAC. Then a BLU-GRN-BLU winding that yields 35.1VAC C/T, and BLU-GRN being 17.4VAC. Seems high if that's the heater winding. The other winding BRN-BRN is 89.3VAC. Maybe that's for the bias supply...126VDC less rectifier loss and load.

I don't have documentation for their model 200S to see how they're dealing with this transformer in their system.

That other transformer that I removed the end bells from....I haven't yet tried to make connection to the BLK and WHT primary wires to see if this xfmr works or not. I'm guessing those primary leads were chopped off for a reason?

I don't yet have any loaded data on the one that lights up. Unloaded, excitation current is 0.31A/30W @ 120VAC. Dimensions of the xfmr is 5.3" wide, 4.5" tall, 5.5" deep across the end bells. Mtg centers are 3.5" x 4.6" Weight is 17lbs 8 oz.

[glebert]

After revising a power outlet box of mine with a mains fuse ahead of the power switch, letting me have protection during powering up off of the variac/power analyzer, I found the HT supply of the other power xfmr to be 524VAC C/T. 524VAC would yield 740VDC less rectifier drop, and this is no load. So, I'd assume these RED-BLK-RED leads to be the HT supply. Then, there's a pair of BLK-BLK leads which is only 26VAC. Then a BLU-GRN-BLU winding that yields 35.1VAC C/T, and BLU-GRN being 17.4VAC. Seems high if that's the heater winding. The other winding BRN-BRN is 89.3VAC. Maybe that's for the bias supply...126VDC less rectifier loss and load.

Steve, thanks again for being willing to check out those transformers. Considering the considerable difference between the voltages you get and what I would need I think I will pass on it. The fact mine has two high (or at least higher) voltage secondaries does seem pretty unique to this amp and other unicorns like the Hiwatts. If I need to add another tranformer to get the screen and preamp voltages or do some sort of circuit bending to get the voltages it may be more than I am willing to put into this project (especially since I don't know if there are other problems).

In an interesting development, the discussion I had with the guy at Mercury Magnetics made me wonder if there could be something going on other than shorted turns. I took the transformer out of the amp and took off the bell cover on the side with the HV secondaries. There was some odd crystallized stuff internal, don't know if that is normal or not. The odd thing is that now I am not measuring any continuity between the HV windings and the core. Still getting the same resistance across the winding. I will post some pics later.

It was also interesting that it looked like the transfomer may have been worked on at some point because it only had three mounting bolts and one was a definite mismatch ot the others (two flat head and one Phillips). Maybe Hartley would do something like that, but it seems odd.