Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JustMike on September 28, 2020, 08:31:52 am
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A couple of questions;
I'm starting a new build- Tubenit/Sluckey's Tweed ODS.
I'm thinking of using the PSU board because of the Bridge rectifier (no CT on the B+ secondary), but that's going to put my B+ at 500v & 490v. I'm going to have to add some more nodes and calculate the dropping values. _OR would you just start over just using the iron?
Then there's the 6v filament winding. I can't find the specs on this xfmr and the original amp was all SS other than the 2x 6L6's. I'll be adding 4 12ax7's and I'm not sure the 6v winding can handle the current.
Thoughts?
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Thoughts?
sell the PT on craigslist for $20
no star_bucks for a week + old PT money = buy a proper PT
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The power supply is already fine for a pair of 6L6-class tubes.
So what is a "Tubenit/Sluckey's Tweed ODS" that it can't end up in a pair of large tubes?
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Looks like it's Tubenit and Geezer's? My Bad.
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A little 6 or 12v PT can run the preamp tubes
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I have a 6.3vCT xfmr. Where would be the best place to put it in this chassis? Closer to the AC switch or run 120v down the chassis to the other side? It's small enough-or the chassis is big enough that it will fit inside.
I also have to keep in mind that I still need to add the rest of the pwr supply nodes.
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Not next to the preamp tube for sure! Put it on the power supply end with all that other noisy stuff. Outside of chassis if possible.
PS... Do I see cable ties on that big ole resistor on the left end of the board? If so, bad idea.
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Yes, those are tie wraps. I put two resistors in series to get 430Ω. Bad idea because of heat?
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So now I'm dealing with the pwr supply. I thought I'd use the Peavey one that I showed above, but it puts out 500v. A little hot for this design. And the filter caps had some tell tale bumps on the ends which I understand to be a sign that they gotta go. So I went ahead and built the Tree design in the schematic. I'm using a bridge rectifier and a 20w 1k resistor to hopefully get the voltage down from 500.
My tech friend advised me that the first cap in the design could be rated higher voltage wise. Because of the turn on surge I assume. What of this?
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So now I'm dealing with the pwr supply. I thought I'd use the Peavey one that I showed above, but it puts out 500v. A little hot for this design. And the filter caps had some tell tale bumps on the ends which I understand to be a sign that they gotta go. So I went ahead and built the Tree design in the schematic. I'm using a bridge rectifier and a 20w 1k resistor to hopefully get the voltage down from 500.
My tech friend advised me that the first cap in the design could be rated higher voltage wise. Because of the turn on surge I assume. What of this?
Forget the resistor. 500V ain't so bad. But if you only need 350V you should get a proper PT. If you are really concerned about 500v we need to see a schematic to get a better idea about your concerns.
Your first and second node caps are rated for 350v, but since they are stacked that amounts to 700V rating. I don't see a problem.
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Sluckey, I think you misunderstood me. I am NOT using the Peavey PSU board but I am using the PT. It's a NON CT 370v, A bridge rectifier gives me 518v (370 x 1.4). My link in the first post has the Peavey PSU circuit. And just to be clear, the amp I'm building is the Tweed ODS. Node A on the Tweed ODS is 430v and the first Cap is a 47u 450v.
Thanks,
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You still need to "stack caps" like peavey did, and I do......
for ~ 15 - 30 seconds there is more than 500vdc as your tech friend pointed out
so look at the psu that you are NOT using and make something real close
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this is something I do before drilling holes.
I ballpark a Load resistor that "simulates" total tube current at idle.
with that I can "dial in" my 1st tap drop resistor, OR call it and buy what I need.
by that I mean, If DropR is 50W it's better to save that PT n move on (the tubeR can be 100w :)
IF you don't understand what I'm talking about after the 3rd read through, DO NOT try anyway :icon_biggrin:
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OK shooter, I'll try;
2-6L6's in push pull=116ma +4 12ax7's@3ma=12ma ==128ma...these are idle current numbers.
Using an online voltage drop resistor calculator I enter the figures 518v and the desired 430v, I get a value of 688Ω with a minimum wattage of 11.3w.
Did I pass?
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it's to early for math :icon_biggrin:
but 12W gets doubled for margin of error, so lots of heat, that said, I've used 25W to "get there"
expect your 518 to be lower some once loaded which will keep some heat at bay.
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Seriously, if you need to shed 88V, then you're just using the wrong PT.
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Yes, I know it's not ideal, but I was hoping to build something out of this old thing. It's just a hobby for me. Can anyone suggest a build based on the iron that I have? Since the donor amp had a SS front end they weren't concerned about the dropping resistors or current in a totem pole PS. I've heard 500 plate volts described as "stiff". I guess this old peavey design was kind of unique so this xfmr may not be good for anything?
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> I've heard 500 plate volts described as "stiff".
I've heard that lobsters who smoke dope taste better. (Around here, lobster matters.)
You can drop the preamp as much as you want without any 11 watt resistor. 100V drop at 10mA is a 10k dissing 1W, use a 10k 5W to be very sure.
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Seriously, if you need to shed 88V, then you're just using the wrong PT.
Ok then. Like I said earlier, this is what I call a boneyard build, I have the chassis & Iron that I'd like to build something out of. Maybe I should look at something with EL34's?
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EL34s is an option. You could build a killer Plexi or a screaming JCM-800. But as PRR said, this iron is already fine for a 6L6 amp. No reason to not build tubenit's tweed ODS. Just don't try to reduce the B+ to the power amp nodes A and B. You can use larger resistors to easily drop voltages for nodes C, D, E, and F to match the Tweed ODS.
Tubenit favors cathode biased power amps. But with voltages so high, I recommend building the fixed bias power amp just like Peavey, including the HV power supply and bias supply. You'll end up with a more powerful Tweed ODS which may sound a bit different from tubenit's cathode biased amp, but nevertheless, a nice amp. If done properly I don't think you will be disappointed.
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Thats what I needed to hear. Thanks again.
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I ordered some new filter caps and while I'm waiting I thought I'd tackle the fixed bias ckt. The Peavey sch shows a fixed fixed bias ckt. Also, I'm using just the PSU board, so everything to the right of the dotted line is on me. For the bias ckt that means a pretty lumpy -76v.
Q1-So to build this bias ckt, I think the Peavey sch is mostly correct, but if I want to make it adjustable do I just come off of the diode into my 10k trimmer ( with a 27k to gnd)?
Q2- I'm assuming that what I see here on the Peavey sch is a non adjustable fixed bias ckt. Any advantages to using this?
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I would do it like this...
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I was considering putting in a standby switch, but because of the PCB I would have to cut a land and insert the switch between the bridge rectifier and a land that connects 3 other points. The new caps are rated at 450v so 900v rating in series should handle the turn on surge. I mean, it worked for Peavey for 30 years, right?
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Right. No Standby switch necessary with 900v rating on the Brute Force Caps. In replacing the cathode Bias ckt with fixed it goes here, correct?
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I got it working. I ended up using the Peavey PSU for the Plate & Screen nodes. I upped the stock 400Ω to a 920Ω. I think it's a 25w (one of those gold plated ones- about 1"long). That put my plate voltage at 484 and my screens at 473. Then I used a Ohmite tubular ceramic 18Ω 15w for my node C dropper and the voltages are very close to what the Tweed ODS schemo calls for.
The Schematic calls for a choke between nodes A&B, but because they are located in the Peavey PSU PCB and Peavey uses 2- 220k R's in series there, I left them. Am I right in thinking a choke here will increase the node B voltage?
Also, per Sluckey's suggestion, I put in a fixed bias ckt and set it at 35mv across the cathode resistors (actually one tube reads 35, the other reads 32 with old tubes).
Bench testing thru an EVM 12l speaker, it sounds pretty good. The clean channel sounds nice and it's quiet. The drive channel is very smooth but noisy (hiss, not hum) when I turn up the PPIMV. I did not wire the fx send & return jacks as I'm thinking about putting in a tube buffered loop like I did on Tubenit's Ryan's Amp. I just wanted to get this thing working first.
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2-6L6's in push pull
quick math says 16-17W/tube
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Hi Guys
Just checked. The original attached Power supply (Peavey Classic VT) shows exactly the same part no for T1 (16732 and 16733) as for the Peavey Deuce and Mace. Could we assume sufficient heater winding for all tubes required and possibly for pair of EL34?
Anyway a quick inspection of diameter of Heater wires should give the secret away. :laugh:
If it is the same transformer as Mace/Deuce, it did supply 6x6L6GCs. So a Pair of El34s (3A heater Current) would be easy.
Note: The export model had a 4A Slo Blo Fuse fitted on Heater winding.
Please see attached Sch.
Kind Regards
Mirek
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Interesting glass54, I was unsure about the heater winding, so on my build I added a dedicated 6.3v xfmr for the heaters. I ended up leaving the original heater winding on the output tubes, but because the Classic VT amp I have had a SS front end I was unsure if it could handle the 4 additional tubes. But it looks like it can- 6x900ma>2x900ma+4x300ma.
ALSO, I was snooping around in the amp and noticed that I'm only getting 74v on the plate of v1a. Schematic says I should be reading 161v. Dropping resistor reads 97k. Cathode voltage is high at 3v. Voltage at node E is correct at 212v. Perhaps a bad tube?
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> getting 74v on the plate of v1a. Schematic says I should be reading 161v. ..Cathode voltage is high at 3v.
Is it the right tube?
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> getting 74v on the plate of v1a. Schematic says I should be reading 161v. ..Cathode voltage is high at 3v.
Is it the right tube?
Interesting question! The schematic I built to shows a 12ax7, but the TOS & tweaks schematic shows a 5751. I'm using a 12ax7.
EDIT- I was using a JJ ECC832 which is a 12DW7. So I learned that this is a dual triode, but the first triode (pin 6) is about 1/5 the gain of the "normal 12ax7". But I still don't understand why I have the voltage drop on pin 1. In any event, when I replaced it with a 12ax7, my plate voltage is fine.
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A low-Mu tube biased as for a high-Mu tube will suck more current and sit at a lower plate voltage.
I had suspected a 12AU7; the 12DW7 is a half-and-half.
The plate voltage may not be super critical. The gain will be less, which may matter.