Champ Reverb Build

[bryankloos]

Hello all,

My good buddy and I have decided to each make our take on the Champ, and compare final results in a few months when we are complete with the builds. It will be fun to see where we each go, and how things look/sound at completion.

So, for my build, I'm thinking about the following:

1. Single ended interchangeable from 6v6, 6l6, el34/6CA7
2. Cathode biased
3. 2 tube reverb with dwell and intensity
4. TMB tonestack with RAW pot/switch
5. NFB with switch/pot
6. 10" speaker
7. Valve rectified
8. CLCRCRC filter for the PS
9. Probably 12ax7 - ECC82 Verb - 12ax7 - 6v6/6l6/6ca7.

I've seen a few schematics out there that would be good starting blocks, for either 6v6 or el34. Is it possible to keep all tubes happy with cathode bias?
I'm guessing a single OT at ~5K would work, though the classictone 5K8K:4/8/16 looks like a good contender. What other iron would be a good option?
For PT, would any PR transformer get the job done? I'm thinking I'll have a few extra tubes compared to the basic champ, and filament current will need to be adequate..

Any thoughts before I start going down the rabbit hole?

Thanks.

Bryan

[M Fowler]

This was a good thread.

Mark

https://ampgarage.com/forum/viewtopic.php?t=30988

[Stevem]

I think your kinda wasting your time by trying to run a 6v6 tube and then the other 6L6 like tubes within the same output transformer in that build!

A 6v6 is looking for a 8500 load to sound like it should and the 6L6 family more like 5000 from the OT so to me it’s using the 6l6 families, or the 6v6, 7408 or 6K6 types.

[bryankloos]

Agreed...

I was thinking about maybe a 5-5.5K primary for both, and the n realized I can probably get something with 2 taps (5K/8K).

Just a thought...

The goal is the higher power of the 6l6/el34/6ca7 so I will focus there if it is not feasible to cover both...

[Stevem]

Well you have brought up another part of the tube issue, that being if you have enough power supply current capacity to get 11 watts out of a 6l6 then you have twice as much current on hand then a 6v6 needs, this in turn means that a 6v6 regardless of the OT load will not give up the goods until kinda late in its output level that it’s capable of,

[bryankloos]

Very interesting...

I'm coming from the opposite background, where I started building HiFi 2 channel amps and preamps. Our design goals were the exact opposite... Ample power reserve and operating in the most linear part of a valve.
I've got to rework my thinking.

This said, can you offer up the sonic attributes of asking the valve to pull more current from the OT that it has available, versus clipping the signal by operating the valve outside of ifs intended range?

This will help me better understand the what and why of things.

Thanks!

Bryan

[JGK]

Hi I'm JGK and new here I've been building amps for about 12 years now and happy to find such a wealth of information here. Just built a Champ Reverb based off the layout here by Telentubes came out pretty nice I did upgrade transformers from Allen amps and used an EL34 . Came out pretty sweet I am tweaking it a it now to fine tuning the reverb and tone caps but I am very happy with it. Plays nice clean but turns to crunch around 4. I will try and post some picture later.

[bryankloos]

Looks great!
Did you follow the schematic exactly?
The more pics the better.
Glad you like it!
Bryan

[JGK]

Pretty much followed the layout beefed up the coupling caps and changed the reverb mix resistor to 2.2 megs. Bias resistor is 400ohm with a 100uf cap. added a choke and changed the 2nd dropping resistor to 1K from 10 to put a bit more voltage on the 12ax7 its running around 240. Replaced the rectifier with a solid state one and it tightened things up a bit , JJ el34 is running at 380 or so and biased around 82% which gives a good balance between headroom and crunch. Played it through a Celestion Gold 10" Speaker today and it was very sweet. So far pretty happy with it. Just ordered a face plate started the cabinet yesterday.

[bryankloos]

Hi All,

Its been a busy couple weeks at work.

I've picked up my iron from David Allen, and should have it in a few days.
I've also decided that I will avoid the 6v6 and focus solely on the 6l6/6ca7/el34 for this build.

I'm going to roughly follow the pre and reverb topologies from the AB763, swapping in a TMB/Raw and Reverb with tone control.

The output will be a cathode biased SE topology with switchable NFB.

4/8ohm switchable on the output.

I may add a MV, tbd..

I'm working up the schematic in the next week or so and will post for critique.

I'm going to build in a blank Princeton sized chassis to accommodate the extra rotary's and and switches.

I look forward to some constructive criticism once I'm finished with the schematic.

Thanks!

[bryankloos]

Insomnia got the best of me last night, and I made some progress on the schematic. I'm still working on the PS values to get voltages where I want them, which in its own right is a moving target.

As mentioned, I'm taking bits and pieces from various sources. Here is what I have so far, and I welcome some critique"

Champ-Verb 6ca7.sch

[pompeiisneaks]

Although sharing the .sch file is always welcomed, not everyone has an app that can read that file, it would also be cool if you could get a screenshot for those that don't have the apps and don't want to install them, so you get more eyes on the schematic.

~Phil

[bryankloos]

Thanks for the suggestion!

Champ Verb 1.1.JPG

[pompeiisneaks]

Looks pretty decent, the only thing I'd note:

1. it looks like you are jumpering two triodes for the input to the reverb transformer, but don't show the connection of the grid of the second half at all...
2. you just show B+ as the input to the reverb transformer, is that the C node?
3. You have your C filter node covering a lot of triodes. It might be wiser to add a D node and use that for the first triode to get a bit more filtering for that most sensitive first stage, and decouple it a bit from the reverb and PI


1 and 2 are more 'notes' 3 is a design option.

~Phil

[bryankloos]

Looks pretty decent, the only thing I'd note:

Thanks! I'll take pretty decent, given its my first attempt at mod'ing something to my personal taste.

To your points:

1 - Yes, pins 2 and 7 are jumpered. I will adjust the schematic.
2 - My intention was to take A the the SE output TX. B to the control screen and the Reverb input V2 (B+ was an error). C is for the reverb recovery V3 and the input/tone tube V3. I will label the tubes accordingly.

I'm thinking the extra CLC will help with help reduce hum with this SE design. I could use some advice on operating points for the valves. I'm hoping to keep V4 at about 85% for longevity as it will be installed in a cabinet upside down. Do you think an extra node would help much given I have the CLC upstream? In theory, C is only serving V1 and V3.

Any other items that you think need addressing?

All help is appreciated!