Clone build Tranny voltage too high

[eddiep1966]

Hello All,
I tried searching this up but didn’t find anything. Sorry if it is related to another post. I’ve been building a Marshall 100watt clone. I use a power tranny and output tranny form a old Kern amp that was loaded with 6550s. I’m using El34s in the amp but my voltage off the power tranny (HT-Winding) is high. When I measure the voltage at the standby switch its 640volts. Is there any way to bring this down to a usable level.

[maxkracht]

Are the power tubes installed? Voltage will be high if nothing is drawing current.

[Phil_S]

Definitely try with power tubes installed. However, I don't think we can expect the voltage drop that you'd like to see. Before we jump to any conclusions or what to do, let's get the basic info. Post the loaded plate and screen voltages.

[martin manning]

I don't think we can expect the voltage drop that you'd like to see. Before we jump to any conclusions or what to do, let's get the basic info.

+1! What sort of rectifier circuit are you using? Is it the same as the donor amp used?

[cdemike]

I don't think we can expect the voltage drop that you'd like to see. Before we jump to any conclusions or what to do, let's get the basic info.

+1! What sort of rectifier circuit are you using? Is it the same as the donor amp used?

+1 and +1. Also, is the standby switch wired as a DC or AC switch? If that's VAC, that's totally appropriate for a brown sound 100w Marshall build if you use a full wave (not bridge) rectifier, which should give you about 450VDC unloaded.

[eddiep1966]

Well put in the tubes and found an issue with the bias supply. With the bias voltage set to its coldest the tube idle at 178mA. Boy did that drop the HT winding voltage Plates 472 volts and screen was 456volts so I think it would be best just get a new tranny then fighting this thing. If I try to continued to use this one I'll have make a voltage doubler in the bias supply to get the mA down then try to get the HT down. It seems like it not worth it.

I'm using four diodes in a bridge circuit with center tap not connected. I'm unsure what the donor amp rectifier circuit was, when I got the donor trannys they was the only thing left on the chasse and finding a schematic on a kern amp like winning the lottery. The standby is wired after the rectifier bridge.

[martin manning]

I’m betting that if your PT was originally wired with a FWB, the CT was connected to provide a screen voltage at half of the plate voltage for the 6550’s. I agree the best option is to look for a more suitable transformer.

[eddiep1966]

Ordered a new tranny so when it comes in should be good to go. Thanks to you all that gave your thoughts
good night.

[LOUDthud]

I’m betting that if your PT was originally wired with a FWB, the CT was connected to provide a screen voltage at half of the plate voltage for the 6550’s. I agree the best option is to look for a more suitable transformer.

If that is the case, then the OT will have a high primary impedance, perhaps 5000 Ohms. Such an amp can get 100W from a single pair of 6550s. How many output tubes did the Kern amp use ?

[pdf64]

Well put in the tubes and found an issue with the bias supply. With the bias voltage set to its coldest the tube idle at 178mA. ...

Key information that you need to take away from ghis is that the relative magnitudes of the voltages at g2 pin4 and g1 pin5 are what controls anode current.
So >600V at g2 will need much more than the typical -35V bias voltage at g1.
Don't just stick valves in, power up and hope for the best.
It behoves you not to wreck your valves.
Use of a light bulb limiter in these scenarios is extremely beneficial, basic good practice really.
I suggest you get yourself sorted out with one while you're waiting for a suitable power transformer.

Have you checked the primary impedance of the salvaged OT?

[eddiep1966]

I do have a light bulb limiter and used it many times. they were only on for less then a second or two and I shut it down because I saw the current shooting up. The reading was what the meter said when I pulled the switch. And no I have not check the impedance of the tranny. Will look into it at a later date. The donor amp had 4 power tubes.

[pdf64]

I do have a light bulb limiter and used it many times...

That's great, but it would have been a really good idea to use it here, how come you didn't?
It's tricky to do things better next time if you don't accept that things could have been done better last time

[eddiep1966]

To make sure I got true readings. The tubes are old tubes I use for just things like this and also I placed 500ma fuse in which blew . I normally don't do things this way and if I was working on somebody else amp I sure would not. There is more then one way to skin a cat.

[eddiep1966]

Thought I'll drop an update on the 100 watt clone, Been working on my house and got behide. Well install the new power tranny and she came alive. It is a beast. Had a slight hum in her after finding it was some poor grounding points on the chasse, redid the ground scheme to star ground and it is dead quite. All voltages are right were they need to be.