Yet Another #124 In Progress--Just Finished.

[David Root]

Have been working on this when I get a chance for about three weeks now. Not a faithful copy by any means, setting up for individual output tube bias, and a bit different style power supply layout.

[Structo]

David, I'll bet you have your mouse configured to open things on a single click.
So when you hit the submit button, it perceives it as a double click.

Check your mouse properties in the Control Panel.

[David Root]

Thanx Structo I think that's it!

[Sonny ReVerb]

That's what I tried to say back in this post: https://tubeamparchive.com/viewtopic.php?p=97459#97459

I wasn't as eloquent as Tom, I guess. :p

[Structo]

Sorry Sonny, I remember your post about it but it dawned on me he was still having trouble.


David, what is the board on the left wall with the dual chips with heat sinks?

[David Root]

That is the main HV rectifier diodes, IXYS 1200V 12A soft recovery hexfreds, on this side of those is the bias supply.

[David Root]

Some progress recently. Built relay boards, wired footswich jack/switches, wired individual bias circuit, built DC supply for 12AX7s. Put them all in so I can see how they match up.

Next is populate the main board.

I'm using funk's assembly method, close as possible, more or less.

[David Root]

Haven't even put the Variac to her yet! No FET in yet, don't have the NTE452 I ordered.

Standard Twin iron, all by Magnetic Components. I also got a M-C 100W Marshall OT in case my predilection for big iron gets the better of me.

Regulated 12 VDC on 12AX7s, three relays. I splurged and bought a quad of 7581As, branded Mesa STR-415, which is what was in #124 when it was photographed.

Full individual bias. That was alot of work, I might not do that again. See the four little bias adjust knobs on the upper chassis view.

[Structo]

Man, fire that bad boy up!

[M Fowler]

Yet another great build Man you guys are way ahead of me.

[dogears]

Congrats Dave! Let us know how those PRP resistors sound! I have some here....

[Structo]

What are the small radial electrolytics by the bias pots?

[David Root]

I took Funk's advice and broke down the 100uF bias cap HAD used into one 47uF right after the diode and then 10uF directly on each of the four bias pot wipers. (40 is approx. 47).

What I'm missing on the bias pots is a 220K 1/4W from the entrance to the wiper. I must say the bias pots seem very well constructed and have a really good feel to them, almost like a PEC pot.

dogears, I don't know that the PRPs have a "sound". They are very quiet and are designed for flat extended response. Really overkill in a guitar amp very likely but the quietness is good. I've been using them for some time now. They are not sold as mil-spec like RN-65s but they are a twenty years newer design than RN-65s so they should be very compatible in Dumble circuits.

[greenbottle]

What brand of pots are you using, what size are they and what value.
I have been reading some of Kevin O'Connor's books lately and he is advocating the same type of biasing.
It never ceases to amaze me. I read something, think about it and lo and behold someone on this forum has got most of the information I need.

Thanks Guys

[David Root]

greenbottle, I am using PEC RV4 pots except the volume and prescence which are Clarostat RV4. All 2W. The prescence is 5K changed to 2K with a 3K3 resistor across the entrance and the wiper.

Fromleft to right, volume 1M, treble 250K, mids 250K, bass 500K, OD ratio/drive 100K, level 250K, master 1M, prescence 2K (effectively). Standard #124 values.

PECs and Claros are great pots, I prefer the feel of the PECs but both are good.

However, my next build is a #13 50W in a Princeton reverb chassis and I just bought some Bourns open case 1/2W pots from Steve Slater at Apex Jr. These are similar to the standard CTS/Alpha pots many use and I picked these because the bigger RV4 size doesn't fit comfortably in the smaller sloped face chassis. I just received them last night and I have to tell you these are very very smooth turning pots that rival the feel of the PECs, and only $1.89 each. If they are as good electrically as they feel physically these are EXCELLENT pots.