Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Willabe on July 14, 2015, 06:41:10 pm
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I'm ready to see if this circuit works as drawn.
Will the plate load R and K R that Gibson has for the 6J5 splitter and 6SN7 modulator work with the 12AX7 I'm going to use? Or do I need to change them (rebias the 12AX7's)?
I have a Champ 5F1 preamp in front. I'll run the vibrato into a 5F1 output stage, just to test it. I'm using Doug's BF/SF Champ/Princeton PT.
(I didn't draw in the filter caps/dropping R's for the B+, I'll put them in as I bread board it as I want to try a few things.)
Here's the GA55V schemo;
http://el34world.com/charts/Schematics/files/gibson/Gibson_GA-55V_true_pitch_shift_vibrato.gif (http://el34world.com/charts/Schematics/files/gibson/Gibson_GA-55V_true_pitch_shift_vibrato.gif)
Edit; fixed layout drawing, missing input jack ground wire.
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I looked up the 6J5 and 6SN7 and both have an amplification factor of only 20.
So I'll start with a 12AU7 1st.
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FYI, the 6cg7 is a 6sn7 equivalent in a 9-pin miniature package. So that might be an option to consider. Gibson themselves used it in the GA83S in a similar vibrato circuit to what you're building. You can probably borrow values from this schematic.
http://www.davidsonamp.com/sg/gschemGGA83sPWRMP.php (http://www.davidsonamp.com/sg/gschemGGA83sPWRMP.php)
Also note the similar circuit in the vibrato channel of the vox ac30. Think they used all 12AU7s.
Good luck, let us know how it sounds! I once built an op-amp and VCA clone of the GA83S circuit (scaled all the values in the phase shifting circuit by a factor of 10) and it's pretty cool sounding.
Nathan
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The 6J5 stage is not critical.
The 6SN7 stage is a delicate balance of hiss and overload. You probably want a low-gain and NON-linear tube. The 6SN7 obviously works. My impression is that the 12AU7 is "like" a 6SN7 but LESS linear. So the AU is not a best-choice sub for SN in "all" applications. But in *this* case I think the AU7 is even better than the SN.
6CG7 is also similar but "bigger", handles bigger power. This is not a power circuit. I see no benefit using the CG, and fear it may not have as much nonlinearity as the AU7.
The bottles are similar enough that no radical bias change is needed; different enough that I would replace the 1.5K cathode resistor with a 1K fixed and a 1K pot, diddle to see if it has a happier setting.
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PRR,
Am I mistaken about the 6CG7 being a 6SN7 in a nine pin package? I swear that I've read that somewhere.
I get what you're saying about a non linear tube being better for this application... ideally, you want something with a remote cutoff characteristic, like a vari-mu compressor, yes?
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Ok, 12AU7 it is and I'll diddle around with a pot on the modulators K's. Thanks. :icon_biggrin:
Nathan, thanks for the info and I will post my results. (I'd love to hear a sound clip of your SS vibrato.)
I am aware of the Gibson GA83S and that Vox lifted it for their vibrato.
Gibson had a good # of mistakes in their schematics across their line, so I don't even know if the vibrato section of the schemo is correct.
If the GA55V circuit works then I'm going to A/B it with the Vox/GA83S circuit and see how they sound compared to each other.
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Am I mistaken about the 6CG7 being a 6SN7 in a nine pin package? I swear that I've read that somewhere.
it is. some differences in max ratings though. the 9 pin variant has a lower maximum plate voltage rating (450V vs. 300V) and lower maximum plate dissipation ratings (5W vs. 3.5W). 6FQ7 is also 6CG7.
--pete
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I think the 6SN7 guts were re-worked to fit in the mini bottle.
Mostly it is either/or, no real difference.
In *this* case, I think he wants the 12AU7. It was "hot rodded" for better performance at high current; conversely performance falls-off at low current. Usually, that's not-best. In *this* case, we WANT the gain to vary with bias to get the wobble. And as 12AU7 is inexpensive and common, why not use it?
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Nathan, thanks for the info and I will post my results. (I'd love to hear a sound clip of your SS vibrato.)
Well, here's a picture of the SS vibrato as it now stands. On the positive side, you know I'm not a bullshitter. On the negative side, you can see just how 'great' my freehand pcb layout skills are! Have to rediscover what goes where and hook it up. Built this about 10 years ago. Uses a TDA1074A stereo electronic volume control IC as a VCA.
(https://flic.kr/p/vREK16)
(https://flic.kr/p/vREK16)
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Looks good, give it a home! :icon_biggrin:
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Setting up the 12.6vac heaters PT for the 12A_7 preamp tubes. It's Hammond 180K12, with dual secondaries.
Secondaries; 1. Red/start, Red/Yellow/finish. 2. Green/start, Green/Yellow/finish.
For 12.6vac, I would stack the 2 secondaries. So I hook up the Red/Yellow/finish with the Green/start as CT for the stacked 2 secondaries? (Then Red/start and Green/Yellow/finish will output 12.6vac.)
This would keep them in phase so as not to cancel each other out?
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Just hook'em up temporarily like that. If you're correct you can measure 12vac end to end. If wrong, you'll measure zero end to end. If zero, just reverse one secondary.
Why are you wanting to heat the 12A?7s with 12 volts? That transformer is only good for 1.19A @ 12v. Is that enough?
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Just hook'em up temporarily like that. If you're correct you can measure 12vac end to end.
OK, easy enough to do.
Why are you wanting to heat the 12A?7s with 12 volts? That transformer is only good for 1.19A @ 12v. Is that enough?
Yes, I believe so. At 12.6 they only draw 1/2 the current so 150mA per 12A_7 x 6= 900mA. I bought the PT years ago on close out from AES, might as well use it. It's toroidal, small and quite.
I hate wiring up 9 pin sockets, crossing the socket to get to pin 9. This way just pins 4 and 5 and I have the sockets oriented so pins 4/5 are facing 180 degrees/opposite/far side away from the barrier strips and the 1/2/3, 6/7/8 wires going to them.
Then I can use the main PT's heaters for just the power tubes. Should be a little neater wiring up things.
I made up a couple of humdinger boards, 1 for the 9 pin tubes and a 2nd for any power tubes, that I'll feed their wipers with an adjustable stand off dcv. Even put a mini chicken head knob on it that I had laying around. :icon_biggrin:
With a toroidal heater PT, humdinger pot with stand off dcv at half the current draw through the heater wires should make for a very quite heater supply.
I'll post pics soon.
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is that that a 182K12? - no such CURRENTLY listed part as 180K12, or perhaps it's an older discontinued model?
usually, a hammond part number denotes that the first three letters is the series, the 4th letter is the VA rating of the series and last remaining digits are the rating of the secondary(ies) - in this case 2 x 12V secondaries. so, if that's the case, then they need to be in parallel not series (stacked), but then again i'm probably totally off the mark here...
--pete
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is that that a 182K12? - no such CURRENTLY listed part as 180K12, or perhaps it's an older discontinued model?
Yes it's an older discontinued model, see post above.
It's 2x6.3vac secondaries at 15VA. (Labeled on the box and PT.)
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never mind...
see attached datasheet.
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so connect red/yellow to green with red to pin 4 and grn/yel to pin 5
black connects to brown to hot
white connects to orange to neutral
--pete
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Seems to be overly complicated just to heat the tubes on a breadboard.
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Seems to be overly complicated just to heat the tubes on a breadboard.
Yes........... but there's a method to my madness. :laugh:
I wanted to play with the stand off dcv to hear for myself what, if any benefit it is to go as high as 70 to 80 dcv as apposed to the usual 30 to 40 dcv. The single ended Champ I'm putting on it should be a good test to start with. (No PP phase cancellation in the output power amp.)
The humdinger pot I also wanted to play around with to see if it's worth the extra trouble/parts/space/expense.
Plus I want to see if using both together is worth it. Seems to sound good on paper but......... how much difference do they really make :dontknow:
It was only 3 little eyelet boards I had to make, used left over cut offs/scrap board.
This way I'll know and put it to rest 1 way or the other.
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so connect red/yellow to green with red to pin 4 and grn/yel to pin 5
black connects to brown to hot
white connects to orange to neutral
Thanks Pete. :icon_biggrin:
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so connect red/yellow to green with red to pin 4 and grn/yel to pin 5
black connects to brown to hot
white connects to orange to neutral
Thanks Pete. :icon_biggrin:
YW... can't wait to see and hear this experiment. :icon_biggrin:
--pete
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Seems to be overly complicated just to heat the tubes on a breadboard.
I wanted to play with the stand off dcv to hear for myself what, if any benefit it is to go as high as 70 to 80 dcv as apposed to the usual 30 to 40 dcv.
Do the test for experiential learning. I think it won't make a difference in the end.
D.C. standoff is most useful *when/if* you have tubes with heater-to-cathode leakage. In such circumstances, half the heater voltage cycle, the heater voltage is negative compared to the cathode voltage. The heater emits electrons which leak to the cathode during those half-cycles when the heater is negative of cathode, and you get half-wave rectified hum in your tube cathode (and through to the plate). Biasing the heater positive always keeps the heater at a higher voltage than the cathode, reverse-biasing this "leakage diode" and stopping current flow between those elements.
{As an aside, I think the super-large value cathode bypass cap in the tweed Bassman was a case where heater standoff wasn't used, and hum due to leakage was objectionable with some tubes, so the bypass was made 10x bigger to make the cathode appear more like ground for all audio, and especially that hum.}
30-40vdc should be enough for most circumstances. Where it might not is if you have a gain stage with the cathode sitting at 45v, 50v, 60v, etc. Maybe like a cathode follower or phase inverter. Now it makes sense to bias those tubes' heaters to voltage higher than their cathode voltage. However, these kinds of stages are usually later in the amp where signal levels are higher and a little leakage might not create enough hum to be an issue. Which also explains why you usually see most of the heavy-duty hum elimination tricks in the first stage or two of the preamp.
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Any progress?
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Yes. :icon_biggrin:
I bolted in and soldered up the 12.6acv PT, it's humdinger pot board and the dc stand off board.
Next the barrier strip blocks for the main PT, power tubes and meters.
Here's an updated block drawing of the bread board set up for a Champ, shows meters.
Edit; Fixed drawing.
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Here's a few pictures;
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More;
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More, this is the acv meter/on/off/fuse box that will go between the variack and BB;
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And;
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Little more done today.
Preamp tubes 12.6acv ACV input 120ACV installed and screwed down.
I had a piece of braided shielding and some 'spaghetti' isolating tubing that I bought a good 10 years ago so I decided to us them. I see no reason not to start to us up these things I bought for just this purpose.
Is it necessary, probably not but, I paid for it and it's just sitting......... :icon_biggrin:
I slipped the braided shielding over the 120acv wires feeding the toroidal 12.6acv heater PT and soldered a ground wire to (only 1 end) to the far right/PT side of the bread board (BB)
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Heres a pic of the barrier strip block I bolted in to take the 120ACV in to the PT primary. I also bought this (these) big barrier strip block's years ago, I want to use them up. I used #8 bolts and added rubber washers because the plastic is a little brittle. I habve broke the corner off 1 before.
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Still trying to figure out how I want to place the rest of the barrier strip blocks.
Here's a pic of possible placement of the rest of the barrier strip block's, the power tube(s)/rectifier sockets din rail and the K current meters.
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Last thing for now, I've been using this for a flux past brush, so far I like it a lot. :icon_biggrin:
It's small, and I can slip it into an eyelet so it applies flux to both sides.
Inexpensive, so you can pitch it when you want, has a cover sheath, so I can cover it and lay it down any where without getting sticky flux paste all over. :icon_biggrin:
When I 1st saw my wife using it for/on her teeth it I instantly said "hey let me see that, I can use that!"
She was not happy and said no! But a tool is a tool. :undecided: :laugh:
(Sorry, I should have put a quarter down to show it's size, it's about 3" long with the cap on.)
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Link for part 2 of this;
http://el34world.com/Forum/index.php?topic=18354.msg187427#msg187427 (http://el34world.com/Forum/index.php?topic=18354.msg187427#msg187427)