Need an idea on DR voltage question

[Luthierwnc]

I am working on a homebrewed DR that I never quite finished a couple years ago. It is a single channel version with no trem so four preamp tubes. All of the values are stock and I am using what I remember as a Weber DR power transformer.

The HV taps are 334 each so that should be right on. Using an old Mullard 5AR4 and a pair of JJ 6v6's, biased at 24 ma, I was getting 390 VDC on the plates. I thought a quick fix to get the B+ up was to use a diode string rectifier. The amp was already a little over-capped at 40/20/20/20 so I left that as is. Fired-up, the plate volts were still around 390. Another pair of 6V6's read slightly higher.

The preamp nodes are reading proportionally low as well but I don't suspect them right now. The stages are plates through choke, screens, 10k to the inverter, 10k to the gain stages. Nothing unusual.

As you'd expect, the amp is mushy at volume. I'd really hate to have to replace the PT. If you have an idea why the volts are getting knocked down so hard, I'd be interested.

Thanks, Skip

[jjman]

390V is 94% of the "ideal" BF schematic voltage. I'm surprised that the diodes didn't increase it. Many people go thru great pains to lower their voltage on 6v6s when it's say 425 or so. Maybe the poor tone is coming from something else.

What is the heater voltage?

[sliberty]

What are you seeing at the nodes?

I have a similar problem on my Express (low voltages), although the tone is wonderful. But I never did figure this one out. So I am particularly interested in your issues.

[Luthierwnc]

The weirdest thing is that the reverb driver knocks 11 volts off the B+. I'm getting 431 VDC at no load. The power tubes alone take it down to 414 which would be fine. The other three preamp tubes maybe drop a volt each. But put a 12AT7 in the driver hole and it brings it down to 400. I'd suspect a parasitic but not with the volume off unless it is just channeling EMF.

I'll get a scope on it over the weekend to check for ghosts.

sh

[Luthierwnc]

This might be two issues. With the driver tube pulled I am getting 411ish VDC. With the driver tube in I'm getting 401ish. The cathode is reading 91 VDC -- even with the grid wire removed. I swapped the 12AT7 for another and the reading was identical. It ought to be 9 volts. The supply is pulled from the "B" node. The reverb is overwhelmingly strong -- even with a voltage divider on the grid (220k/750k). The K resistor/cap is the stock 2k2/25 and is fine.

So, 411 isn't as good as 420 but I could probably live with it. Two, what would make the driver tube run away with no signal?

Thanks, sh

[Andy Le Blanc]

measure the driver transformer..... could it be on the way out....

[Luthierwnc]

I had a spare reverb transformer and changed it out last night. Same story. The voltage on the cathode is 94VDC with no signal. The cathode resistor and cap measure right on the nose.

Unfortunately, my DMM fried something inside and the ohmmeter part is malfunctioning. I'll see if I can get another one today.

[Luthierwnc]

I had a spare reverb transformer and changed it out last night. Same story. The voltage on the cathode is 94VDC with no signal. The cathode resistor and cap measure right on the nose.

Unfortunately, my DMM fried something inside and the ohmmeter part is malfunctioning. I'll see if I can get another one today.

[Andy Le Blanc]

so maybe not the driver......
so ...... bad filter cap?.......rectifier?......... power tranny?

or may be...... if the the coupling cap that supplies the driver is bad or is too
large it might suspect ..... or the grid resistor of the drive tube might not be grounded?

more straws to reach for

[Luthierwnc]

Yep! It was at the "huh?" stage.

I unsoldered everything, put it back together and it works fine. There must have been a short somewhere. Experience has taught me that in all likelihood, the problem is something sloppy. It is my version of control-alt-delete.

Power tube plate volts come in at 415 which might be a tad low but it sounds like it ought. Definately a keeper. I have an ADA 50 watt output transformer in there. If I had a little more respect for the power transformer I'd probably drop in a pair of 5881's.

Thanks again, sh

[Andy Le Blanc]

had some of those myself......

[ChrisM]

What taps of the Weber transformer are you using?

The 540V AC or the 680V AC?

On my DR built I used the 540V AC and pulled my hair out for the longest time trying to figure out why my B+ was 398V DC (not under load). Finally switched to the 680V AC and got a B+ of 496V DC (not under load). Pissed me off because I was told by the 540V AC winding would work.

My power tube plate voltage is in the 430V DC range I believe. So right on spec and the amp sounds great.