68 Power

[jnew]

Combing through some specifics on this next build. I've seen the diode in the bias circuit as 1N4003's and 1N4007's. What importance may this offer?

56K bleed resistors on power caps. Amps with power cap boards underneath don't seem to have/use them. Amps with the caps on top of the chassis (69 onwards) do have them. What's up with that?

From the PI to the power tubes, I've seen 5.6K's to each tube, one 5.6K between each set of OPT tubes and even one 1.5K between each set.

What say you folks?

Not gonna use 1 ohm to grounds. Don't think the early 68's had these.

[Tazin]

Most of the things you listed were done by Marshall to increase circuit stability. The '68 100w amps mostly used a 10D8 for the bias circuit which would be basically equivalent to a 1N4006 diode.

[jnew]

1N4006? Hmm. Haven't seen that any schematic. Where can I see the 10D8 equivalent? How did you find out about that one?

[Tazin]

A lot of people who have drawn these schematics have actually never seen a real plexi amp let alone reseached them enough to know things like this.....Most folks just re-hash someone elses work to make life easier.

As for the 1N4006 being basically equivalent to the 10D8....Google the specs and see for yourself.

[Strat78]

I like the look of these, pretty sure they would work, thoughts?
http://www.ebay.com/itm/1N2069-GI-VINTA ... 5ae92f27d5
-Robert carries similar ones in the store. James, I know you know all this, but I'll just go over it for the sake of new builders: There is the whole discussion using faster "fred" (UF4007) for the bias vs. the regular 1N006 to 008's. Rob made a comparison clip way back when comparing fast vs. slow diodes. Then we can also go with the larger 3amp diodes for the bridge. For a classic 68 I go with the two 1.5k's for the swamps, but I always variac the wall voltage no more that 110v to prevent extra stress on the amp. The "flying" 5.6k's are the safest, but nah. The 1ohm resistor to ground is the easiest and safest way to bias, but once you get the hang of biasing the old way it looks more vintage just doing a bus to ground off the sockets. -Some say it sounds better that way too.

[jnew]

Phil, yeah man I'm with you. I did my 68 Iskra build only the two 1.5K's. But what's interesting about that is that there is nothing between the 220K's on the board and valves 5 and 6. The 1.5K's are only between V's 4 and 5, and then 6 and 7. Hmm.

Definitely no 1 ohmers to ground and amp will never go above 110VAC.
As far as the FREDS, I don't think they were around when plexi's were built so I don't really consider them. I'll check out the link.

[Strat78]

Something about if V5 or V6 fail, V4 or V7 won't get toasted. Are the extra 5.6k on V5 and V6 redundant?
So you are doing a NOS build!

[jnew]

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(861.38 KiB) Not downloaded yet

Yeah, pretty everything but those Riken RMG resistors.

[rgorke]

I kinda like the way those blue resistors look.

[jnew]

Yeah, definitely some cool looking carbon comp resistors. 1% tolerances too, so should be interesting with stock values.

I hadn't thought about it until just now but with only one 1.5K between V's 4&5 and one between 6&7, I can pull the two inner tubes, play and listen, and then put those same outer tubes to the inner position and give it a listen. That way if those resistor actually affect tone, I can tell right away without tedious soldering. I've always wondered how they might effect tone.

[vh junkie]

About the 56ks that you see across the screen cap in some versions... AFAIK those are there to form a voltage divider that keeps equal amounts of voltage across each of the screen caps. Without them, if one cap starts to go bad, one of the caps might have its rating exceeded (e.g. 350v). Those Rifas are probably 450v rated so, likely wont be an issue. You see lots of pics where folks have added them to original 12 series for this reason. I think I remember SDM saying that the 56k value was sort of an arbitrary value. Some builders use 100k. Don't need these across the mains caps though...

[jnew]

Funny you mention 220 values there. David (plexified) e-mailed me about that as well. Gonna leave them out though. Just love doing these things old school. Your explanation on this was awesome man and I really appreciate that. I'm assuming they don't make any tonal changes.

[Ralle]

The sound difference between 5.6k and 1.5K... well, it kinda steels from the top freqs with a higher value... But at the same time; I've tried the same a/b swap between 4/7 and 5/6 socket, and having the tubes in 5/6, makes it sound more ( or should I say farty, or too open ) open in both lows and highs... the snubbers keep it together, one might say... the higher a value, the "kinder" it sounds... I belive the 50 w amps don't have anything there at all... the 220k's goes directly to the tubes...

[jnew]

Those are great findings and definitely in line with pre-conceived notions I was having about all that. Out of curiosity, what did you finally settle on. The two 1.5K's in total?

[Strat78]

Found these on ebay. They look just like the R10D81 that a took out of StarGuitar's 68 plexi, but these are D6's, would they still work?
http://www.ebay.com/itm/International-R ... 339d56b273