Developing the DIY "Champ" for first time builders

[RJ Guitars]

I added a 25uf bypass cap to the 2nd gain stage and it really started to come to life.... but not ideal yet. I think I should have added it to the first gain stage but I'm going to leave it at as is and start prepping some chassis now. I'd like to get a few of these out to you and and let everybody join in the joy of tweaking them into submission.

rj

The published 5F1 schematic shows both triode stages unbypassed, but all the factory-original 5F1s I've seen had a 25uF across the first-stage 1K5 like on the 5E1. I didn't think to keep notes whether they were early or late By putting the cap on the second triode, how much negative feedback do you lose?

I am not sure what the loss of feedback is, do you any thoughts about the best way to determine that?

I am considering taking a couple ideas from "Merlins" book on preamp design and using a 25uF across the 1.5K cathode resistor on the first stage then going with a 1uF bypass on the 2nd stage.

I couldn't find a voltage chart for the 5F1 but I do have one for the 5E1 which apparently uses the same power tranny. Right now my voltages are really close to the 5E1 values.

I had assumed that the 5F1 was newer than the 5E1, but it kinda seems like they took a step backwards if that is the case. The sound I got with the 5F1 circuit was cool but I had to use the bridge pickup on my Stratocaster to get enough treble for it to be useful. I got a lot more treble once I added that bypass cap.

I should mention that it isn't necessarily a lack of brightness as much as it might be a dominance of lower frequency response. Still early in the process of sorting that out but it is getting down to the fun stuff now...

rj

[M Fowler]

I have this newer Bronco schematic that has voltages, can't be a whole lot of differences.

[Firestorm]

I added a 25uf bypass cap to the 2nd gain stage and it really started to come to life.... but not ideal yet. I think I should have added it to the first gain stage but I'm going to leave it at as is and start prepping some chassis now. I'd like to get a few of these out to you and and let everybody join in the joy of tweaking them into submission.

rj

The published 5F1 schematic shows both triode stages unbypassed, but all the factory-original 5F1s I've seen had a 25uF across the first-stage 1K5 like on the 5E1. I didn't think to keep notes whether they were early or late By putting the cap on the second triode, how much negative feedback do you lose?

I am not sure what the loss of feedback is, do you any thoughts about the best way to determine that?

I am considering taking a couple ideas from "Merlins" book on preamp design and using a 25uF across the 1.5K cathode resistor on the first stage then going with a 1uF bypass on the 2nd stage.

I couldn't find a voltage chart for the 5F1 but I do have one for the 5E1 which apparently uses the same power tranny. Right now my voltages are really close to the 5E1 values.

I had assumed that the 5F1 was newer than the 5E1, but it kinda seems like they took a step backwards if that is the case. The sound I got with the 5F1 circuit was cool but I had to use the bridge pickup on my Stratocaster to get enough treble for it to be useful. I got a lot more treble once I added that bypass cap.

I should mention that it isn't necessarily a lack of brightness as much as it might be a dominance of lower frequency response. Still early in the process of sorting that out but it is getting down to the fun stuff now...

rj

5F1 is newer than 5E1, but it seems like they changed their minds about having the bypass cap on the first stage. I know I've had Champs from as late as 1960 that had it and they were labelled 5F1.

The feedback question is based on the fact that you're bringing it to a cathode that is bypassed by a big enough cap to shunt most or all of the feedback to ground. Thought it might be audible. The blackface circuit has the 1K5 on the second stage bypassed with a 2uF, but hangs a 47R below that and brings the feedback in at the 47R/1K5/2uF junction. Feedback R is 2K7.

Brightness/chimeyness etc. is way dependent on the OT and the output tube, so you may want to tweak the circuit for the performance with the components you've speced. I've always found that with the bypass on the first stage, you don't really need it on the second, but you could try putting the conventional feedback on the second stage cathode and using a really small cap across Rk, not so much as a bypass but as a fixed presence control.

BTW, published 5F1 voltages are:

B+1: 340
B+2: 295
B+3: 250

[RJ Guitars]

Very Good Stuff, thank you! I have moved the bypass cap over to the first stage and I'm getting ready to give it a listen.

thanks again,

rj

[RJ Guitars]

I added a 25uF bypass to the first stage and also added a 1uF to the 2nd stage... I know I shouldn't make two changes at once.

Now it's treble city and I was immediately looking for the tone knob on the guitar. I'm going to pull the 1uF off of the 2nd stage tomorrow and give it a listen again.

Simple changes, huge differences. I was getting concerned about the output transformers but I am relatively pleased with the possibilities now.

I did knock out one chassis this evening before I started tweaking on the amp. After building one prototype I have moved almost everything but only slightly from my last layout drawing. I also talked with Edcor today and they are expecting to ship transformers by the the first of next week.

Looking forward to watching these come together.

rj

[xtian]

RJ, can you say the dimensions of the chassis for this project? Thx.

[Firestorm]

I also talked with Edcor today and they are expecting to ship transformers by the the first of next week.

Looking forward to watching these come together.

rj

The Edcors are the "upgrade," right? What are the specs?

[RJ Guitars]

RJ, can you say the dimensions of the chassis for this project? Thx.

12 x 6 x 1-3/4

Also - How about a drawing with dimensions on it? This is a super simple cabinet but fits with the minimalist idea of the Champ style amp.

This is not the cab design that I plan to use myself or offer up for others. I am going to use one with a little more volume and a slanted front panel.

Sorry for skinny lines on the drawing. My drawing program likes to think of lines as being infinitely thin... You have to open up the drawing in it's own window to see the lines.

enjoy,

rj

[RJ Guitars]

I also talked with Edcor today and they are expecting to ship transformers by the the first of next week.

Looking forward to watching these come together.

rj

The Edcors are the "upgrade," right? What are the specs?

I am curious how big these will be. Every time I order what I think is a small transformer from Edcor it usually turns out to be about twice as beefy as I thought it would be.

Upright Power Transformer with End Bells
120V 60Hz to 460 volt (230-0-230) @ 60mA
6.3 volt (3.15-0-3.15) @ 2.5A,
5 volt @ 2A

Upright Output Transformer with End Bells
10 Watt Plate Z 5000 ohms (40%)
to 8 ohms Secondary
Freq 40~18KHz w/leads
Single Ended Transformer

rj

[TheGimp]

The opt sounds like the GXSE10-8-5K but upright instead of low profile.

If so, you could flip the drawing to be vertical and it should work.

http://www.edcorusa.com/DWG/15000-EI087-100-HX.pdf

I'm curious about the PT as I've been using one with the slightly lower ratings (50 vs 60mA) w/0 the 5V winding made by Hammond and sold by Allied with their name on it as 6K49V for $22 and change. If the Edcor is close in price I may switch to it.

[RJ Guitars]

The opt sounds like the GXSE10-8-5K but upright instead of low profile.

If so, you could flip the drawing to be vertical and it should work.

http://www.edcorusa.com/DWG/15000-EI087-100-HX.pdf

I'm curious about the PT as I've been using one with the slightly lower ratings (50 vs 60mA) w/0 the 5V winding made by Hammond and sold by Allied with their name on it as 6K49V for $22 and change. If the Edcor is close in price I may switch to it.

No that is the same output tranny. I meant upright rather than drop in the hole but it is the low profile part number that you have above.

The PT is a custom wind that I gave them the specs for. I originally had it at 75mA but when I asked for the 5V tap @ 2A they told me that I was over the power dissipation rating for this size of tranny. I also had to bring the 6.3v tap down from 3A to 2.5A so we could get all three windings. I don't think they actually use smaller wires on the secondary or heater taps but rather just change the numbers on the label to keep it within the overall rating for that tranny. I'm not sure what their safety factor is either but I suspect 60mA is conservative.

rj

[TheGimp]

I have this newer Bronco schematic that has voltages, can't be a whole lot of differences.

Man! Those voltages have to be wrong.

24V at the cathode over470 Ohms is 51mA bias.

342 V on the plate with 51mA bias is 17W dissipation!

Also, there appears to be 78V across the OPT primary, at 51mA that's 4W dissipation.

342V at the screen, follow the wire and it shows 410V at the cap for a drop of 68V in a section of wire!

365 AC at the transformer gives 516 peak minus 5Y3 drop and filtering with a 40uF cap should be around 410 or so. Therefore, it appears that the cap voltages are correct.

It just does not make sense.

[Firestorm]

I have this newer Bronco schematic that has voltages, can't be a whole lot of differences.

Man! Those voltages have to be wrong.

24V at the cathode over470 Ohms is 51mA bias.

342 V on the plate with 51mA bias is 17W dissipation!

Also, there appears to be 78V across the OPT primary, at 51mA that's 4W dissipation.

342V at the screen, follow the wire and it shows 410V at the cap for a drop of 68V in a section of wire!

365 AC at the transformer gives 516 peak minus 5Y3 drop and filtering with a 40uF cap should be around 410 or so. Therefore, it appears that the cap voltages are correct.

It just does not make sense.

A lot of Fender schematics are "strange" on the power amp side (better down at the preamps). Sometimes the AC input to the rectifier makes no sense at all wrt the reported B+. The Bronco is theoretically identical to the Vibrochamp: same circuit, same trannies, etc. Yet the Vibrochamp schematic says it gets 315-0-315 from a 125P1B, while the Bronco thinks it gets 365-0-365. In the Vibrochamp, B+ is 355V rather than 420V. Plate of the 6V6 is still 342V, but doesn't have to drop 78V(!) across the OT. VC also shows only 21V being developed across the 470R cathode resistor, so only 44.6mA. 44.6 x (342-21) = 14.316. Much more reasonable.

I've never understood how some of the wacky numbers get on these schematics.

[Zippy]

I am curious how big these will be. Every time I order what I think is a small transformer from Edcor it usually turns out to be about twice as beefy as I thought it would be.

Upright Power Transformer with End Bells
120V 60Hz to 460 volt (230-0-230) @ 60mA
6.3 volt (3.15-0-3.15) @ 2.5A,
5 volt @ 2A

Upright Output Transformer with End Bells
10 Watt Plate Z 5000 ohms (40%)
to 8 ohms Secondary
Freq 40~18KHz w/leads
Single Ended Transformer

Anyone have a read on how well this combo will work with 6L6/EL34/KT66/KT88? I've looked 'round for spec sheets for the various tubes and am coming up short for spec's regarding running in SE mode.

How much current do these tubes draw at this voltage?

How large an output tube will this OT handle?

[RJ Guitars]

I've had a couple inquiries now about a Super Size version of this. I am on the verge of ordering a half dozen sets of iron but I think they may require more real estate than the little brother... any thoughts?

I was thinking:

PT
230-0-230 @ 150mA
6.3v @ 3A
5v @ 2A

OT
15 watts SE

rj