custom circuit -- proofreading needed --

[mort]

Checking arounf the ef86 I found the 220k resistor was actually 22k resistor that was reading 45k. Got rid of it and put in a 220k and still getting a similar voltage drop. Here's what I have, reading directly off the ef86 socket...

pin 1 (grid 2) 5.92vdc
pin 3 (cthode) 61.4vdc
pin 6 (plate) 6.46vdc
pin 8 (grid 3) 61.4vdc
pin 9 (grid 1) -320.6 vdc

how could I possibly be getting that kind of voltage reading on the input grid? I can see how that would cancel out positive voltage but I don't see where it's coming from. Wiring looks coorect so far... gonna pull the tube and read voltages again/....

edit: with the ef86 pulled there is:

pin 1 10.4vdc
pin 3 0vdc
pin 6 9.2vdc
pin 8 0vdd
pin 9 0vdc

[MarkusBass]

Can you post a photo of the circuit? This looks like you have still mistakes in the assembly. Either ground is missing, or you confused anode with cathode (or other pins of the tube). When you have 9V on the anode with pulled out tube, you should immediately check what voltage you have on the other side of the 220k anode resistor. Do you have power supply at all for this tube? Or maybe you confused 22k power supply resistor with 220k anode resistor. Or you connected 8uF capacitor in reverse. The possibilities are endless (especially in the beginning of the year).

Mark

[mort]

Wait a second...

I made an error. The resistor where I'm getting the massive voltage drop is the 22k coming off the B+ rail and then feeds into that 8uf filter(I used a 10uF). I took the 220K back out of there and put in a new 22k. Without any preamp tubes installed at all I'm still getting the voltage drop and the resistor began to cook. Now I'm really confused

[mort]

Can you post a photo of the circuit? This looks like you have still mistakes in the assembly. Either ground is missing, or you confused anode with cathode (or other pins of the tube). When you have 9V on the anode with pulled out tube, you should immediately check what voltage you have on the other side of the 220k anode resistor. Do you have power supply at all for this tube? Or maybe you confused 22k power supply resistor with 220k anode resistor. Or you connected 8uF capacitor in reverse. The possibilities are endless (especially in the beginning of the year).

Mark

Will do.. gimme just a sec

[Malcolm Irving]

Looking at the voltages with the EF86 pulled, pin 3, pin 8 and pin 9 voltages are as expected (i.e. zero).
However, pin 1 (grid2) and pin 6 (plate) should both be up at B+. The plate should have a 220k resistor to B+ and a 1.5nF cap going to the vol. control. Pin 1 should have a 1M resistor to B+ and a 0.1uF cap to ground. When the tube is pulled there is no current through it, so no voltage drop from B+ to those pins.

[Malcolm Irving]

.... Without any preamp tubes installed at all I'm still getting the voltage drop and the resistor began to cook....

Maybe you connected ground to the wrong end of the 8uF (or 10uF) cap?

[mort]

BAHH -- I did in fact install the 8uF capacitor backwards. I'll still post the pics in case that's not my only problem.

[mort]

[MarkusBass]

So it seems that if you had measured the voltage on the other end of the 220k resistor, you would find the problem immediately and save half of the day. I wonder why you didn't do it .

Mark

[mort]

So it seems that if you had measured the voltage on the other end of the 220k resistor, you would find the problem immediately and save half of the day. I wonder why you didn't do it .

Mark

I did, I just didn't understand where the voltage was going. I'm not nearly as seasoned as some of you so sometimes I don't see things that are second nature to most here.

I do suss out many of my own problems. Those occasions just don't end up as threads

[mort]

Tried getting a replacement cap from the local surplus but they had none rated high enough so I just removed it for now. The amp is working mostly normal now, need to get some shielded input wire to fight off some interference. Also getting some high pitched noises at certain(high) eq settings and also when the guitar's tone pot is near max. I'm guessing these may also be related to input interference.

I always get confused with bias.. right now my max setting on the bias pot yields 57mA of plate current per tube(used a 20k pot instead of 10k). I'd like for the swing to be able to get up to ~72mA and dial it back to around 60mA. Would I replace the 27k bias resistor with a higher value or add say a 5-10k in series to achieve a wider swing?

V1- NOrmal ch input
1. 239
2. 0
3. 1.9
6. 199.9
7. 0
8. 1.5

V2 EF86 Ch2
1. 91.5
3. 2.7
6. 98.8
8. 2.7
9. 0

V3 ch2 12ax7
1. 194.6
2. 0
3. 1.5
6. 374.6
7. 194.0
8. 204.9

V4 12at7
1. 216.3
2. 33.7
3. 103.0
6. 215.8
7. 33.3
8. 103.0

V5 6L6
3. 425.4
4. 424.8
5. -31.1

V6 6L6
3. 422.5
4. 422.4
5. -31.1

And reading across 1 ohm resistors on the power cathodes there is 57.7mV and 57.8mV at max bias.


Should the power grid resistors be a little higher in value to get some margin between the grid and plate voltages?

[g1]

I always get confused with bias.. right now my max setting on the bias pot yields 57mA of plate current per tube(used a 20k pot instead of 10k). I'd like for the swing to be able to get up to ~72mA and dial it back to around 60mA. Would I replace the 27k bias resistor with a higher value or add say a 5-10k in series to achieve a wider swing?

Did you make any of the bias changes suggested in posts #25 or 26? Kind of dangerous not to.
Maybe post the bias circuit schematic.

[mort]

Did you make any of the bias changes suggested in posts #25 or 26? Kind of dangerous not to.
Maybe post the bias circuit schematic.

Yes. I always tie one end to the wiper. I am hip to that safety practice.

This is what is installed in the amp. Same as the schematic and also shows the wiper tie in and the correction for the 20K pot.

[g1]

You want more idle current, so you need less negative voltage on the grids. So decrease the 27K. That will shift the whole range, so if you still need to be able to bias with low idle current, you would also change the pot to a higher value.

[mort]

For anyone interested here is a clip of teh amp and there are more on my soundcloud account. There are clean and gain channel clips using a 335, Strat, and Tele. All the gain channel clips are into the lo input with teh volume at 4 or so and the drive control at 7. All of the clean channel clips are into the hi input with the volume at high noon.

https://soundcloud.com/mortatone/mor...n-gain-channel

There's still some noise riding on the hi input so we didn't record with it today. Would putting a pF cap in jut the right spot somewhere possibly help with this issue?