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Planning for an all-octal build

Discussion in 'Shock Brother's DIY Amps' started by laird, Dec 9, 2010.

  1. laird

    laird Tele-Holic

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    So, 3M's "clear" full sheet labels are anything but clear, they're basically ricepaper with glue on the back. I ended up using regular 2" wide shipping tape and doing two pieces on each face, with about a 1/4" overlap centered over the pinstripe. There are some air bubbles under the tape but i can live with that.

    So the amp is 99% assembled now. I just have the two mains wires to go and have to experiment with the bias supply to get the right voltage. Testing will start tomorrow night!

    -Laird
     

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  2. Cliff Schecht

    Cliff Schecht Tele-Holic

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    Looks GREAT inside! My only comment is that having the choke not be at a 90 degree angle to the PT while having it so close will couple some 60 Hz noise in. The reason we orient all of the transformers at exactly 90 increments from one another is so that the stray fields mostly cancel out and reduce hum. It's an old ham radio trick that we still use today.
     
  3. laird

    laird Tele-Holic

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    Thanks Cliff! Yeah, the choke fitment wasn't ideal... it was a compromise I had to make to fit into this little chassis and cabinet. I'm hoping the 7 henries will help compensate for the layout issue. :)

    The good news: the lightbulb limiter paid for itself again!
    The bad news: 0.2 ohms DC resistance (give or take DMM accuracy) across the 5V winding isn't good. When a rectifier is installed it would pop a fuse instantly if there wasn't a lightbulb in the way.

    I'm waiting for word back from Edcor on if I can pull the bellhousings and look for a short myself or if they just want me to ship it back.

    -Laird
     
  4. laird

    laird Tele-Holic

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    I'm an idiot. I had the 5V winding center tap grounded. Lifted it and used the CT as my HT output and it worked great.

    So once I fixed that, I realized some solder had flowed down into pin 7 (filament) on a power tube socket. After a few attempts to draw out the solder I gave up and clipped the pin. While tapping it out of the base with a jeweler's screwdriver, the octal base snapped and now it can't hold pin 7 at all. I don't have any spare octal sockets on hand, so now I'm waiting for the mailman to deliver a rush order of 4 octal sockets.

    Once that's solved it's time to work out the bias supply. I decided to feed it from an extra secondary tap (260v I believe) since it was sitting around, but I have to reduce the dropping resistor to compensate for the difference in voltage.

    -Laird
     
  5. marshman

    marshman Poster Extraordinaire Silver Supporter

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    I'm just curious, laird, but is there anything a well-equipped hobbyist would be able to do with a shorted PT? Even if you'd opened it up and found that was the problem, wouldn't it still have needed to go back to the source for repair, or can you fix that sort of thing at home?
     
  6. Cliff Schecht

    Cliff Schecht Tele-Holic

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    Not an easy fix by any means and IMO not worth the money or time as Edcor will guarantee their products. I don't know if Edcor uses interleaving on their power transformers but even if they don't, you would have to figure out the specs of the transformer before you did any winding. I doubt Edcor sent out a bad transformer but an easy test is to put it on a low voltage function generator set a 60 Hz sine wave @ 1.17V RMS (or 1.2V RMS if your transformer is wound for 120V) and see that you get the "proper" voltages from each leg of the transformer (divided by 10 of course!). You can test it on a variac too but if something is bad in there, it will probably fry quicker than you can turn off the variac. I always test with a function generator first, it's cheap insurance. If you do use a variac, try to install a really low value fuse (100 mA or less) as any short will cause this to trip and save you some smoke and more frustration.
     
  7. laird

    laird Tele-Holic

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    So I got the broken octal socket replaced this weekend, got the bias voltage in range and got her fired up this weekend with some 6L6s. All the operating voltages are right on target, and the sound is clean and full. The relocation I did of one cap in the 1974 tone stack did screw up the behavior. The tone all the way down shunts the whole signal to ground... and I wired the tone pot backwards. :) Fixed that in the latest design.

    Here's the trouble: I'm getting almost no gain at all from the preamp stages! Channel 2 is roughly double the volume of channel 1, but either one would get trounced by a Champ. At full volume I'm seeing a max of about 1VAC coming out of the coupling caps... I'm expecting 25-30v. Touching the PI input with the multimeter probe makes the expected LOUD static scratch/pop noise, so I'm pretty sure the PI is running correctly. I've tried swapping the preamp tubes around, and I get essentially the same results (PI OK, preamp lacking gain) in every case. Next I'm going to build a 9-pin to 8-pin adapter so I can sub in known-good 12A*7 tubes to the circuit.

    I've attached updated schematic and layout... There's got to be something I've messed up that I'm just not seeing... a wrong pin or something...
     

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    Last edited: Jan 10, 2011
  8. FenderLover

    FenderLover Poster Extraordinaire

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    What are your B+, plate, and cathode voltages for Channel 1??
     
  9. laird

    laird Tele-Holic

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    For channel 1, the cathode voltages were 1.1v (low triode) and 1.5v (high triode). I think the B+ was a little over 300 and the plates around 220v.
     
  10. FenderLover

    FenderLover Poster Extraordinaire

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    It looks to me like that tube is starved for current. A 12AX7 substitution may fare better, but I reject the idea that the 12AX7 and 6SL7 are interchangeable with the same circuit values. It's like dropping a 12AU7 or 12AT7 into a 12AX7 spot thinking the pins are compatible, just less gain. Does it work? Probably. Does it sound good? No. That's because the AT an AU idle up with 10 times the current than a 12AX7 does.

    Lets fix this. 300V B+ and 220V at the plates leaves 80V across 100K=0.8mA current for BOTH triodes. Starvin' Marvin. Idle it up to 2mA for each triode. 220V for the plates is a nice number (a little lower is OK), so the 100K should to be 22K or 27K.

    Trust me on this: change the cathode resistors to 680 or 820 each. Start there and see what you get. You will never get 25-30V out of this either. I know the mu=70, but you'll never see it. You'd completely overdrive your PI and output sections anyway. With .25Vp-p in, I'd guess ~8V out of one (with bypass cap) and ~6V out of the other. It's still probably enough to drive the 6V6's after the PI.

    There you go - Three resistors. Try the 22K/820 set or 27K/680 set. W'dya get?
     
  11. laird

    laird Tele-Holic

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    Ah, I found a bad solder joint on V1 and the input jacks to V2... Here's what I have now (still with the 100k plate, 820 and 2700 cathode resistors):
    B+: 347v
    V1 pin1: 0v
    V1 pin2: 146.7v
    V1 pin3: 1.15v
    V1 pin4: 0v
    V1 pin5: 146.7v
    V1 pin6: 1.54v

    B+: 347v
    V2 pin1: 0v
    V2 pin2: 194.8v
    V2 pin3: 2.4v
    V2 pin4: 194.8v
    V2 pin5: 347v
    V2 pin6: 197.5v

    B+: 394v
    V3 pin1: 87.8v
    V3 pin2: 274v
    V3 pin3: 125.4v
    V3 pin4: 88.4v
    V3 pin5: 271v
    V3 pin6: 125.4v

    B+: 429v
    V4,V5 pin1: .043v
    V4,V5 pin3: 425v
    V4,V5 pin4: 426v
    V4,V5 pin5: -45.7v

    It sounds a whole lot better, but the gain is definitely short of driving the power tubes into distortion. I'm thinking that I'll have to ditch the 1974 pre for a two-stage arrangement. For channel 2 I might try a cascode using the 6SN7... if my math is right it should net around 90-130 mu, and with a 6SL7 in the PI it might just be enough for some overdrive, even on EL34s. Back to the drawing board.

    -Laird
     
  12. FenderLover

    FenderLover Poster Extraordinaire

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    Well, that's a different picture, but not by much. There are only two stages that are biased correctly, so why not get this working instead of going to a different circuit?

    The first stage came up in current. I'd go higher, but this will work. The second stage is still starving on 0.5mA. The lower inputs: first stage is starving on 0.3mA. The cathode follower looks great at 2mA. (I hope you have a 1W cathode resistor there).

    I built an octal Bassman-type amp that drove a 6L6 output very nicely with the same basic gain stages: gain > paraphase> 6L6 and it rocks. Yours should easily drive 6V6's with a LTP you have shown.

    With a new B+ of 347V and a plate of 146V, change the 100K to 47K and both cathodes to 470. You'll be surprised. My point is to get it working, then tweek it. When you have a tweed mixer, you are also sharing bias. Balance the two and see what you have first. If you abandon it without looking at the obvious problems, you'll just have a bad experience and nothing good to say about the glorious 6SL7 :) Again, it's two resistors..... then look at your lower input stages.
     
  13. laird

    laird Tele-Holic

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    I set up a simple 2-stage on channel 1 and a cascode on channel 2 using the 6SN7. They're working well, but it seems that when I overdrive the PI (now a 6SL7) I'm getting into grid conduction or something, because all of a sudden i go from 10VAC on the power tube inputs to 60VAC+ on the inputs with blatty, nasty distortion. I'm out of time for now; gotta fly out on business in the morning, so this project will be on hold till next week. Thanks for your help FenderLover!

    -Laird
     
  14. M Fowler

    M Fowler TDPRI Member

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    The amp looks great and nothing says tone like an octal preamp.
     
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