Champ Reverb Build

[bryankloos]

Progress.... The chassis is drilled front/back and sockets. I need to cut out for the PT, and figure locations for RT, OT and choke.
The layout is complete and I'll start drilling the G10 for turrets tomorrow. Its going to be a rainy day so good times to get some more work done!

[bryankloos]

First time making a turret board. I'm happy with it!
Hardware gets mounted tonight!

[bryankloos]

and a little more progress...

[bryankloos]

Hello All,

Probably premature, but I have a question on my build. I'm finished with the board and have most of the power circuit wired up. I've tested the output of the tube rectifier at 347Vdc. Strangely, the SS rectifier is giving me an output of 287Vdc. These voltages are taken at the base of the rectifier tube, and the common/last diodes on the SS rectifiers at the TBD resistor (currently a 150/2W). There are no downstream connections to C1 or the choke yet...

Does this seem off? Am I using the incorrect SS rectifier configuration? Its built with 1N4007 in I believe a Full Wave CT configuration...

Thoughts?

Thanks.

[sluckey]

It's all good. Until you connect a filter cap to the rectifier all you have is rectified AC pulses and your meter can't figure that out, so the meter kinda averages the pulses and the resulting display is low. Connect that first filter cap and it will charge up to the peaks of those pulses, providing a smoother dc voltage and the meter will give a much better display.

[bryankloos]

Yep, you were correct. With the filter caps and choke I'm about 416 vdc with all tubes conducting other than the output tube.

I still need to wire in the switchable cathode caps and resistors for the output tubes.

What is a realistic plate dissipation for both the EL34 and the 6L6? I want to bias for both good "longevity" and also sound. Is 70% a good goal?

Thanks.

Bryan

[sluckey]

Single ended Class A usually runs 90% to 100%, some even higher.

[bryankloos]

In pentode mode?
How long will the tube last at 90-100%

[bryankloos]

While I'm still waiting for shielded wire for input and vol pot, I did finish wiring up the amp and have initial voltages to report.

V1 A
158v
1.327v
V1 B
155v
1.35v

V2
272v
2.66v

V3 A
163v
1.34v
V3 B
163
1.416

V4
342v Plate
271v Screen
18.01 Cathode (currently 300 ohm)

Id like to see a few more volts on V1 and V3, but I'll wait for you all to chime in.
Initial thoughts?

Thanks,

Bryan

[bryankloos]

Hey All,

Just an update.
I have the amp working, and with initial listening I am baffled by a few observations.
First, I have little low end authority. I'm listening through a 12" Cannabis Rex and the Low E just sounds anemic/fuzzy.
Secondly, I'm distorting quickly into rotation of the vol pot. I wired in 4.7K NFB which has helped, but I'm seeing what I think is asymmetric cold clipping on V1B.

So, I'll double check the TS is working and wired correctly first. Then I may drop the plate resistors on V1B to 81K and re-bias more towards center.

Lastly, I'm thinking about lowering the 10K dropping resistor in-front of the suppressor to get the screen a bit closer to the plate voltage on V4. Right now these voltages are pretty far apart.

Oh, I also need to decrease the reverb mixing, as I have WAY too much reverb. Thoughts on how to do this effectively?

Here are the recent voltage measurements with the updated schematic below (both tubes represented in voltages).

I'd love to hear some thoughts on my thoughts?

Thanks!


6CA7

B+A - 343.5
B+B - 234.5
B+C - 220.3
B+D - 213.1

V1/1 - 133.8
V1/3 - 1.079
V1/6 - 137.3
V1/8 - 1.107

V2/1&6 - 232.7
V2/3&8 - 2.02

V3/1 - 140.2
V3/3 - 1.14
V3/6 - 141.0
V3/8 - 1.19

V4/1&8 - 13.5
V4/3 - 341.1
V4/4 - 224.5


6L6

B+A - 348.3
B+B - 271.1
B+C - 254.3
B+D - 245.7

V1/1 - 160.5
V1/3 - 1.24
V1/6 - 157.7
V1/8 - 1.26

V2/1&6 - 266.9
V2/3&8 - 2.33

V3/1 - 161.2
V3/3 - 1.31
V3/6 - 162.4
V3/8 - 1.36

V4/1&8 - 16.44
V4/3 - 338.6
V4/4 - 264.0

[bryankloos]

Well, I made some major improvements last evening.

I dropped the first resistor in the PS to 1.5K from 10K.
Valves are V4 - 6L6, V1,3 - 12ax7, V2 12at7

B+ is now:

A - 339
B - 328
C - 304
D - 294

V4 with 200 ohm on the cathode
Plate - 319
Screen - 307
Cathode - 18.09

This should get me to about 87-88% MPD. Best I could do with the resistors on hand.

This change also increased the voltages on V1 V2 and V3

V1
189
1.47
187
1.43

V2
313
2.75

V3
191
1.51
194
1.55

I now have MUCH more clean headroom, with distortion staring at about 6 on the volume pot, as measured by my ears. The TS is much more responsive, and the Raw seems much more responsive in a pleasing way. I'm still missing a little bottom end but I'm guessing that will tighten up when I get the speaker in a proper cab.

150 ohm 20W cathode resistors were ordered to get the MPD a bit higher on the EL34/6CA7 side. I'm going to try to get both output tubes closer to 95% MPD.

I still need to confirm the TS is correct and take some measurements with the scope, but I feel like major progress was made.