Modified #124 Schematic for review

[mauiboy81]

Hi Guys.

I am working out a schematic for a 2 6l6 amp based on #124. I've marked some possible changes and was hoping I could get some feedback from the community. I appreciate your help.

I didn't draw it but I am also thinking of including a switch for the mid cap with a .047uf or .05uf.

Thanks to all you guys who have made this type of thing possible.

Nick

[Structo]

The two mid caps should be .047 (or .05uf) and a .01uf.

Feedback resistor should be 4K7.

I have 3K3 grid stoppers on my 50w.

[mauiboy81]

Hey Tom,

Thanks for the response.

So for the mid switch I was thinking of using a spdt on-off-on and having a .01 wired normally so in the off position it's .01 and then I could switch inanother .01 or .04 in parrallel. That way I'd get .01, .02, or.05.

I wasn't sure about the negative feedback and was thinking the resistor needed to be smaller to make up for less being fed back due to the lower power of the 2 6l6's.

Does that make any sense?

Thanks again

Nick

[Structo]

Well nothing is etched into stone on these amps.

There are enough differences in the transformers people use to make it necessary to adjust a lot of the values.

In the end it's what sounds best right?

That's a good choice on the mid cap switching.

That way you should be ok for single coil and humbuckers.

On the choke, I bought a Hammond (194B) from Angela that was supposed to be equivalent to the 022966 and it is 4H @ 90ma 100 ohm.
So not sure what's up with what I got.
I measured the DC resistance on mine and it was 100 ohms.
On some of the ODS schematics it shows the 022966 being 4H 100 ohm.

Be aware, depending on what the transformers actually put out you may have to adjust the dropping string resistors.

[wjdunham]

On the subject of the grid stoppers, I have the 3.3K's on my DLite, #124 has 5.1k's. Is this resistor value dependent on whether the power amp has two or four tubes? How critical is the value, and what function does the resistor perform? Is is similar to the grid resistor in a triode, where it sets the cutoff frequency of the LP filter formed by the grid input resistance and the Miller capacitance of the triode?

I'm curious because I'm building a 100W 4x6L6 somewhere between the tweaked Dlite and #124, and I'm not 100% sure how these resistors affect the sound. I'm loving the sound of the DLite, and I want to come as close as possible to that with the 100W version.

[heisthl]

I can't hear a difference up to around 8k2. A higher value buys you a little more PI headroom because if you work the output tubes hard enough they want to draw some grid current and it's harder for them to do that through the larger resistors. Sometimes I think the Fender designs went for minimum circuit requirements or they assumed a lower value was cheaper

[ayan]

I can't hear a difference up to around 8k2. A higher value buys you a little more PI headroom because if you work the output tubes hard enough they want to draw some grid current and it's harder for them to do that through the larger resistors. Sometimes I think the Fender designs went for minimum circuit requirements or they assumed a lower value was cheaper

I think if you can't hear it, it's good. The only time this made a difference in my life was in a friend's amp, a Marshall that originally had 6550s and had had the sockets rewired to accept EL43s. The amp originally had no (power tube) grid stoppers, and after the EL34 conversion it developed this lower frequency oscillation thing... You could make the amp sound like a Moog synth by turning the master volume. I went as high as I had to with the grid stoppers to get rid of that problem. I guess you could keep going up in value to do a bit of custom voicing, but I would probably rather do that in the preamp -- as opposed to the power amp.

Cheers,

Gil

[mauiboy81]

Hey Gil,

Thanks for all your great insight into these amps, 124 especially.

I have a couple (probably silly) questions about your amps from a couple pics you posted.

I noticed your 1k and 1k5 screen resistors, are your amps set up for el34's?

What are those resistors to ground on your power tube sockets, at pins 1 and 8, I think?

What size resistors do you have across your 220uf filter caps? I have 270K but have seen 220k and am wondering what the difference will be.

Your preamp tubes are reversed form 124, what is the reason for this?

What size resistors do you use for the LNFB on v1? Do you go for the full 40M? Being color blind I have a hard time with the bands.

Do you wire your relays like 124?

Have you ever tried a cut/contour control? I kind of like the idea but am not sure if it is really worthwhile.

Thanks for all your help?

Nick

[ayan]

Hey Gil,

Thanks for all your great insight into these amps, 124 especially.

I have a couple (probably silly) questions about your amps from a couple pics you posted.

I noticed your 1k and 1k5 screen resistors, are your amps set up for el34's?

What are those resistors to ground on your power tube sockets, at pins 1 and 8, I think?

What size resistors do you have across your 220uf filter caps? I have 270K but have seen 220k and am wondering what the difference will be.

Your preamp tubes are reversed form 124, what is the reason for this?

What size resistors do you use for the LNFB on v1? Do you go for the full 40M? Being color blind I have a hard time with the bands.

Do you wire your relays like 124?

Have you ever tried a cut/contour control? I kind of like the idea but am not sure if it is really worthwhile.

Thanks for all your help?

Nick

Hey,

See if this helps:

1. I set my amps for use with both types of tubes, so I wire the power tube sockets accordingly. I use 1K 5W cemet block screen grid resistors and 5.1K grid stoppers in all amps. The resistors you see connected to ground at the power tube sockets are 1 ohm, to measure cathode voltage across 1 ohm, and hence cathode current (which is close enough to plate current).

2. I use 270K with the 220uF caps. The difference wouldn't be that large, but 220K would load down the power supply a little but and make the time constant (RC) of it a little faster.

3. Preamp tubes reversed? Well, I was born in Uruguay, which is in the southern hemisphere. Really, I always wired the preamp tubes the same way. Dumble did it both ways at different points in time.

4. I use two 22 Meg resistors in series for the non HRM amps' NFB.

5. I never wired the channel switching relay like Dumble did in the old days, where he would not ground the input to the OD when in clean mode. I always grounded it in all my amps. Dumble had (no pun intended) to start doing that when he went to the HRM architecture, as the switching is different.

6. I tried a PI treble cut control in a BM, to see what it was all about. I didn't wire it permanently to the amp, however. Never tried it with a non HRM amp.

Cheers,

Gil

[Structo]

5. I never wired the channel switching relay like Dumble did in the old days, where he would not ground the input to the OD when in clean mode. I always grounded it in all my amps. Dumble had (no pun intended) to start doing that when he went to the HRM architecture, as the switching is different.

Really?
Can you expand on that Gil?

I don't think I have ever read about that before.
How do you ground the input to the OD tube?

Edit:

I may have answered my own question.
Moss did this layout a while back and it's the newest at the Brown Note site.

I guess what it is showing is that when the OD relay is not energised the OD entrance is connected to ground.
Never really thought about that.

[mauiboy81]

So I built it. It sounds pretty stinking good. Clean is wonderful! The od has got some hash that I want to work out but even still it is killer.

Some changes from the schematic I posted:

-Twin PT 330-50-0-330

-modern bias supply but with 1k and 15k resistors

-Relay supply with separate transformer and bridge rectifier

-100uf series filter caps with 220k

-dropping string is 3k, 18k, 2.2k, 150k fet sim (I had these values on hand)

-150k 1W CF slope

-Voice switch that parallels either a .01 or .033 (or .047 I can't remember) with the .01 mid cap

-2 x 20meg LNFB

-330pf od snubbers

-od treble bleed with 250kb trim and .001 cap to ground (on the schematic the trimmer is on the ground side)

-30pf MV cap

-no loop filter (250pf/220k)

-100k/110k PI 10k trim

-3.3k GNFB off of the 4 ohm tap

-1uf eletrolytic presence cap

-bias test points with pins 8 and 1 tied together to ground.

I think those are all the changes to the schematic i posted.

Off the top of my head the voltages are approximately 185-195v on v1, 198-208v on v2, 295-305v on v3, 440-455v on v4 and v5 depending on the fluctuation in the line (i've measured 116-122vac at different points in the day)

jj preamp winged c power

42mv v4 and 45mv v5 across the 1ohm resistors

These are some of the problems I am facing:

The hash in the od.

I can not get the side of the PI with the 110k to get above the side with the 100k even with the trimmer all the way. With the tube in there now I can get it within 2v

I wanted to be able to use 6v6's so I put in JJ's and had them at 18-19mv across the 1 ohm resistors and they worked fine for a little bit but then started blowing fuses. So with the 6v6's it now blows fuses immediately but it is fine with 6l6's.

I think that is it, at least all i can think of right now. Any thoughts?

Thanks for all your input and help! You guys are great!

Nick

[Structo]

I would suspect that one of the JJ 6V6 tubes has a short.

Recently I have read of people having trouble with JJ tubes.

[mauiboy81]

Hi Tom,

I tried a set of chinese tubes in there and the fuse was blowing with them too. I don't know the specs on those though, maybe they cant handle the higher voltage.

Nick

[mauiboy81]

I'll post a pic when i get a chance. I don't have the ability to do sounds clips or else i would post some of those as well.

[heisthl]

I can not get the side of the PI with the 110k to get above the side with the 100k even with the trimmer all the way. With the tube in there now I can get it within 2v


Nick

Don't be that concerned with the PI plate voltages going one way or the other Some amps sound better with the opposite plate being higher and it depends on the tube. Here's the baseline I shoot for: