Excessive current draw-possible bad multi section cap?

[quayhog]

I just built an 18 watt inspired amp based on Mark Huss' 18 Watt-enstein circuit. The amp sounds very good, no ground loop noises etc. But.

When I put a 12AX7 in the preamp slot the approximatly 300vdc drops to about 60vdc. This is with no other tubes except a rectifier. 100k ohm shared plate resistor and 1.5k kathode on each triode half.

Could a new 4 section capacitor gone bad be the cause? The wiring on this part of the amp is good and true to the schematic. I've disconnected the tube from the power amp section of the amp with the same results. I get the same results with the triode run in parallel or as one half only. I've tried several different triodes with the same results.

My troubleshooting has eliminated the power amp & PI, the grid section of the triode and the plate and kathode circuits. What else is there to check?

Jonathan

[quayhog]

I built this amp in a Weber Deluxe chassis. This set many constraints and workarounds. I like this kind of puzzle.

The power transformer was cannibalized from an old hifi console. it ran two 6BQ5 tubes a 5Y3 and numerous signal and radio reception tubes.

Since the chassis has three octal holes and two nine pin holes I elected to use a 5Y3 rectifier, a 6DZ7 power tube, 6SL(N)7 in the PI, 12AX7, and EF86. All NOS.

The top panel of the deluxe chassis has two volume holes and a single tone hole. I elected to use a top cut like the traditional Vox circuit.

I used a 4 section multi-cap can. 40/40/20/20 450volt for filtering. the 40 sections are wired into the 6DZ7 plates and screen. This section is seperated by a choke. The 20 sections are wired to the PI and and one section to the 12AX7&EF86. I have another new one from the same batch I could exchange if necessary.

I had a tweed cabinet and 12" speakers.

The only part I didn't have on hand new (unless noted above) was the 150 10 watt chassis mounted kathode resistor.

The OP transformer and choke were purchased from Dave West and are suitable for an 18 Watt or AC 15 build.

Jonathan

[passfan]

Are you saying your B+ off the rectifier drops to 60 volts or the plate of the first preamp tube ? list all voltages and perhaps a pic.

[quayhog]

with a tube inserted in the socket I measure 342 on the B+ node for the preamp. On the 12AX7 plate I measure 63VDC with 2.1VDC at the Kathode.

Without any tubes (except 5Y3) I measure 380vdc at the preamp node and 375 vdc at the plate. Zero voltage on the kathode.

There is no mesureable AC ripple. There is no groundloop noise. The amp does play and sounds reasonably good. Suprisingly so with this issue.

Jonathan

[quayhog]

Currently the preamp is disconnected from the amp except the power supply.

The grids are disconnected and the preamp is disconnected from the amp by disconnecting the circuit at the volume pot input.

It is currently just Plate and Kathode.

I've tried three different tubes in socket with similar results and measured for leakage across the socket pins. I may replace the socket for grins to eliminate it as a possible culprit.

[Bob-I]

I had something similar once. A friend built an amp from a kit and he was not only getting this dramatic voltage drop but also a bad hum. Turned out to be a single strand of a lead shorting.

Now that you've narrowed down to just this tube socket, I'd check all the connections and components with a meter. Make sure you check that there are no shorts by testing resistance between pins on the tube socket.

A good way to measure the connections is to put one of you meter leads on the tube socket, the other on the far end of the resistor connected to it. For example, connect one lead to pin 8, the other to ground and you should measure the resistance of the cathode resistor.

[Structo]

Are you absolutely sure you have the tube socket wired correctly?

If you are sure the tube is good, that is about the only other thing that can be wrong unless you missed a ground on one of the cathodes.

[Richie]

Are you absolutely sure you have the tube socket wired correctly?

If you are sure the tube is good, that is about the only other thing that can be wrong unless you missed a ground on one of the cathodes.

wiring to the 100ks is correct? seems like something is miswired, or wrong value resistor on the cathode. be nice to have a pic to look at what you have going on.

[PCollen]

Currently the preamp is disconnected from the amp except the power supply.

The grids are disconnected and the preamp is disconnected from the amp by disconnecting the circuit at the volume pot input.

It is currently just Plate and Kathode.

I've tried three different tubes in socket with similar results and measured for leakage across the socket pins. I may replace the socket for grins to eliminate it as a possible culprit.

If your control grids are disconnected, you are essentially running the 12AX7 segments open-loop without any control to throttle their quiescent current flow. You are drawing 1.4mA (2.1/1500) of current through each 12AX7 segment, for a combined 2.8mA current through the shared 100K Rp, and therefore dropping 280 Vdc across that 100K Rp leaving about
62 Vdc (382 - 280) on the plates.

The math works...connect the 12AX7 grids to their input resistor network and see if things get better.

[quayhog]

I've tried grounding the grids directly and through a 1M resistor. That didn't fix the issue. I replaced the .1 cap that that ties the plate to the volume control with a new one. Now I get 170 VDC on the plate. This reading is with no other tube installed (except 5Y3).

I'm not sure why disconnecting the originally installed .1/400v cap wouldn't have revealed itself as the culprit. I think I had a high resistance to ground on the plate side through the turret.

The amp has now been running on the bench for an hour with all tubes installed through a load resistor. The grids have been reconnected. My voltage readings are:

EF81
pin 1 84vdc (g2)
pin 2 0 (shield)
pin 3 1.7 (k)
pin 6 76vdc (plate)
pin 7 0 (shield)
pin 8 1.7 (g3)
pin 9 0 (g1)

12AX7
pin 1 108vdc (p1)
pin 2 0 (g1)
pin 3 .8vdc (k1)
pin 6 108vdc (p2)
pin 7 0 (g2)
pin 8 .8vdc (k2)

6SN7
pin 1 59vdc (g1)
pin 2 138vdc (p1)
pin 3 84.5vdc (k1)
pin 4 60 vdc (g2)
pin 5 135vdc (p2)
pin 6 84.5vdc (k2)

6DZ7
pin 1 0 (g1)
pin 3 242vdc (p1)
pin 4 248vdc (screen common to both pentodes)
pin 5 0 (g2)
pin 6 241vdc (p2)
pin 8 9vdc (k common to both pentodes)

heater running at 6.4 vac
rectifier heater at 5.2 vac