EF86 TMB Modern Classic (New Project)

[foreverstrung]

The plate voltage on V1a looks like the 100k resistor is shorted. Overal voltages are pretty high. Start with lowering the voltage at node A and work your way down from there. But FIRST check that plate resistor on pin 1 of V1!!!!

Lower at node A with a Zener diode or 2??

[foreverstrung]

I replaced the 100k plate resistor. The old one soldered in place gave a high reading, like 138k, so removed it and then it read fine removed from the amp. Like 99.8k. I measured a new one and soldered in place with no difference. Still high voltage @V1P1 and elsewhere.
So next I measured, the 27k wirewound and it read 28.9k, so I removed it and replaced with new one. In place it read 28.9. When I removed it it read 26.9k……….
Chasing down this voltage issue, should I go backwards from V1P1 and if so, when do I , if ever, have to remove the cap/resistor to get an accurate reading?
Thx

[JMPGuitars]

I replaced the 100k plate resistor. The old one soldered in place gave a high reading, like 138k, so removed it and then it read fine removed from the amp. Like 99.8k. I measured a new one and soldered in place with no difference. Still high voltage @V1P1 and elsewhere.
So next I measured, the 27k wirewound and it read 28.9k, so I removed it and replaced with new one. In place it read 28.9. When I removed it it read 26.9k……….
Chasing down this voltage issue, should I go backwards from V1P1 and if so, when do I , if ever, have to remove the cap/resistor to get an accurate reading?
Thx

If a resistor is reading high, that means the voltage at the connected part would likely be lower than expected. You're looking for a resistor with a much lower value than expected, to produce a higher voltage than expected. All you need to do is look at the schematic for the B+ rail to see what is what. Also check your cathode resistors to make sure they're correct.

OTOH, a faulty tube could also cause voltage issues. Try swapping tubes and see if the voltages look different.

[foreverstrung]

So I was having a moment with the EF86 channel on this amp. TMB channel is spot on, but the EF86, not so much.... Found out with time what the problem was. I had a 680ohms resistor at pin 1, the grid. Supposed to be 680K. Big diff. I didn't have a 680k resistor so I put three 221k resistors in series to get close, just to see if I was on the right track. The voltages got better and the sound is pretty awesome. Output is strong, clean and no unwanted noise..... I'm hoping the voltages on V2 P1 and 6 will balance out with the right resistance. I've got the correct resistor ordered.
So attached is the current voltage chart. Appreciate any feedback. B+ is higher than I'd hope. I put a zener diode at the A+ to bring the voltage down to where it should be, and it is, but B+, as I said, is still high
.
Also, I'd appreciate clarification on the Bias calculator to figure dissipation. [url]https://robrobinette.com/Tube_Bias_Calculator.htm/url]
I get the top part, tube bias. I can enter and calculate, but going down to "Tube Dissipation Using Cathode Resistor Voltage Drop", I'm not clear on what info to enter to calculate dissipation. appreciate some help here too.
Sound check coming soon.

EF86 votage chart.jpg

[JMPGuitars]

B+ doesn't drop before the Zener. You use the Zener to drop the voltage following the Zener. So you need to shift the OT CT from A to the other side of the Zener, like I show in the docs:

zeners.jpg

Once the Zener is in place, you consider the post Zener voltage as A (where the OT CT should now be connected).

Do you have it wired like the image above, or do you need to move the red OT CT wire?

tube-diss.jpg

Based on the above numbers, and assuming you're using a 180 ohm cathode resistor, I would guess you haven't moved the red wire from the OT CT to after the Zeners?

Thanks,
Josh

[foreverstrung]

Attached is how I've got the zener diode wired. Hopefully you can read my chicken scratches marked up.

IMG_2026.jpg

[JMPGuitars]

So you're at 346.9VDC where the OT CT is connected? That's good if correct.

If that's the case, you need to increase your cathode resistor. If you're using 180Ω, then try 200 or 220 Ω.

[foreverstrung]

So I had to play around with the cathode resistors and zener diodes to get my voltages in the right levels. I ended up going from 180 to 200 to 220v cathode resistors and was never getting the sweet spot. I added a second zener diode and went back to the 180v cathode resistor to get my tube dissipation to 88%. voltage now at A+is 338v. B+ is 319v.
Extremely happy with the sound. Both channels. Each is unique. The TMB channel has such a sweet "shimmer" in the sound. The EF86 is cleaner. Low to no gain. I'm going to see how it takes a pedal this morning. I've been playing around with it all week. Just straight/clean thru the amp. No pedals.
I'll send clips later today.

[JMPGuitars]

Post your voltages. I'm curious about this one. The EF86 should have some dirt.

[foreverstrung]

Post your voltages. I'm curious about this one. The EF86 should have some dirt.

I've got no complaints. Really like what I'm hearing coming out of the box

newest voltage chart.jpg

[JMPGuitars]

I think your socket is still wired wrong. Pins 3 and 8 on the EF86 are supposed to be connected, and should both show exactly the same voltage. There's no way that could pass a highlighter test and show a voltage discrepancy.

It may sound good now, but it will sound even better when it's correct.

Thanks,
Josh

[foreverstrung]

I think your socket is still wired wrong. Pins 3 and 8 on the EF86 are supposed to be connected, and should both show exactly the same voltage. There's no way that could pass a highlighter test and show a voltage discrepancy.

It may sound good now, but it will sound even better when it's correct.

Pins 3 and 8 are exactly the same voltage and are connected. Both show the same voltage. This voltage chart on V2 (EF86) P3 and P8 both have 1.2v

[JMPGuitars]

I think your socket is still wired wrong. Pins 3 and 8 on the EF86 are supposed to be connected, and should both show exactly the same voltage. There's no way that could pass a highlighter test and show a voltage discrepancy.

It may sound good now, but it will sound even better when it's correct.

Pins 3 and 8 are exactly the same voltage and are connected. Both show the same voltage. This voltage chart on V2 (EF86) P3 and P8 both have 1.2v

Oops. You're right, I mixed up V1 and V2. So you need to fix V1. Check the 680Ω resistor at pin 8, and the 1uF cap. One of them is likely shorted.

Anyway, you should be able to get dirt on the EF86 channel, though you probably have to crank the gain a bit. I would check the resistor values for the 680K and 68K resistors. Or just enjoy it if you don't want the extra dirt.

Either way, V1 should be corrected.

[foreverstrung]

Check the 680Ω resistor at pin 8, and the 1uF cap. One of them is likely shorted.

Checked cap and resistor in place and they looked good. I rechecked the voltages and V1 and P3 and P8 are reading as they should be. P3 1.2v and P8 .7v......just moving thru too quick.
Thx

Post your voltages. I'm curious about this one. The EF86 should have some dirt.

Regarding this, it actually does have some "dirt" with increased volume. Just much easier, for me, to get that cleaner sound I'm sometimes looking for with this EF86 channel than the TMB channel. The EF86 channel is not as "touch sensitive" as the TMB channel. In my opinion.....and I like it!!!

[JMPGuitars]

Check the 680Ω resistor at pin 8, and the 1uF cap. One of them is likely shorted.

Checked cap and resistor in place and they looked good. I rechecked the voltages and V1 and P3 and P8 are reading as they should be. P3 1.2v and P8 .7v......just moving thru too quick.
Thx

Well, nice work then! Where's the demo?

[Bieworm]

Well, nice work then! Where's the demo?

Yeah! Where is it?

[foreverstrung]

EF86 channel https://youtu.be/SgQUG6N1HFg
TMB channel https://youtu.be/83BKtxcw__M

[JMPGuitars]

That sounds great. I can definitely hear the dirt there waiting for you to push it (on both channels). Do you have a boost pedal, or a guitar with higher output pickups?

[foreverstrung]

That sounds great. I can definitely hear the dirt there waiting for you to push it (on both channels). Do you have a boost pedal, or a guitar with higher output pickups?

I only had the volume of both channels only up to 5/6. I can push it more......For pedals, I've got an Analogman SunFace and a Keeley 4 knob compressor. Both would boost in there own way....I had until recently a Analogman King of Tone. That pedal was awesome. But I'm a 62 year old carpenter raising his 3 grandkids in SE Texas. We've got a very big house so I can "open it up" at times, but I really had no need for that K of T. As nice as it is.
I use a delay/echo a lot. (Right now, Mad Professor.) With that compressor, it sounds pretty awesome. Phase shifter too. But no boost with it.
Seriously tho, I'm loving this amp, both channels, with no pedals. Really love the sound. The TMB channel has an awesome shimmer to it.......I build a lot of guitars. Assemble them anyways. Repair. Refinish. I've always stuck to humbuckers. I've built dozens of strats. Looking for a good fit. Tone.With HBers, P90's and SC, but never seem to find what I've been looking for. Awhile back a friend asked me to set up his strat. It had the DG20 EMG p'ups. Loved it, so I put a set on a USA strat body from the mid 90's and a 1989 USA Fender neck....This is my #1 player right now.....I've got a few other guitars, but this one is always where I go these days. Finally a winner

[chaliapin]

That amp sounds great!