HRM V2b voltage drop?

[Luthierwnc]

Hi All,

I'm working on a preamp that has both a non-HRM and an HRM Overdrive circuit. The clean and non-HRM work quite well in terms of tone, voltages and waveforms. The HRM is giving me some grief so I thought I'd ask here. To set the stage, I'm using SBE coupling caps, RN resistors on the plates and CF60 metal film resistors on the cathodes. Generic 4.7uf bypass caps -- they work.

When the HRM is switched on, the waveform and voltages are pretty good going into the V2b grid. However, there is a huge voltage drop on that plate. It goes from almost 240v when it is idle to 170v in use. The voltage on the cathode resistor goes from 1.95 to 3.5 volts. The waveform after the 121k resistor is clipping very hard. All this is on a 1k sine wave coming into the amp at about 26 millivolts. I've got the trimmers and pots setup fairly neutrally. The power supply is pretty stiff and V2a only drops about 10 volts with a proportionate cathode voltage increase. 9k5 to ground from the entrance pot wiper.

With a guitar it has a weak, nasal tone. The normal overdrive can be set almost indistinguishably from the regular 102 amp I'm testing this on so I know the theory works. I've switched tubes and the results were the same. The couplers don't seem to be leaking. I'm using 180k/2k7 resistors on V2a and 121k/1k8 resistors on V2b. Everything mirrors the HRM circuits we have all seen and used here on TAG. 500p treble, .02 mid and bass with a 47k slope resistor.

I don't think this is a parasitic but I'm stumped on why that half of the tube is laboring. I'm glad to go get scope shots if it helps the discussion. Here is a link to a different discussion with the full schematic:

https://tubeamparchive.com/viewtopic.php?t=23241

Thanks, as always, and I look forward to your comments, Skip

[ToneMerc]

Skip, HRM's give me fits as well. What's your as bullt, is it the "hrm working preamp" diagram?

Mike

[Luthierwnc]

Hi Mike, Yes, it is cut and pasted right from the files with the exception of a 47k slope resistor on the HRM stack. I've attached a more recent version with a couple small changes in the preamp and a split power supply.

I suspected the power supply first wondering if it couldn't recover but the other half of the tube is relatively unphased by being flipped in and out of the signal path. The regular OD doesn't sag much either. But a 65 volt swing on a 12AX7 tells me something is pretty wrong. The temptation is to just stuff a 180k resistor in place of the 120k and see what happens. It could be a parasitic or a bad shield but the waveform is still OK on the grid. A little warped but still a sine pattern.

BTW, if you looked at some of the comments on the original thread, I'm not seeing a "too clean" problem of not sagging the whole amp. At least for the parts that work. Obviously, the preamp is sagging too much but if my history is any guide, it is operator error. I'll see if I can't post some pics after work tomorrow. sh

[Luthierwnc]

Hi Mike,

In all their glory.

The first shot is the gizmo itself before I put in a new relay board. The second is what the non-hrm looks like as it feeds into the loop. All of the controls are set around 12 and the entrance is around 24k to ground at the wiper. The third picture is of the V3a right after the .01 coupling cap. Considering it has just gone through its third gain stage, it still looks pretty wholesome.

The next three shots will be in the next post

[Luthierwnc]

Next on tap is what the signal looked like on the V3b grid, after the 500p treble cap and after the .022 mid cap.

[Luthierwnc]

Finally, this is what it looks like at the loop out -- again with most of the controls around 12 and 9k5 on the entrance wiper.

Compared with the non-HRM loop at the same input signal strength this is pretty weak. This shot is on the most sensitive setting I've got (5 mv). The non OD shot is at 50 mv per division.

What do you think is sucking the life out of V3b? At the V3a coupler behind it the signal is strong like bull.

Cheers, sh

[Luthierwnc]

Faulty rail design. Here's how it looks now and it works fine. There is still some sag from quiescent to signal but the latter is at the right voltage. Thanks for looking, sh