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j allen shaw
Junior tube assistant
Posts: 5
(2/28/04 10:43 am)
5F8-A Twin Project - Flabby/Farty Bass?
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I'm starting a new 5F8-A project and am inviting input from anyone who could help. I'll be using the original 5F8-A Twin schematic, but want use 6V6's instead of the 5881's.
I'm gutting a Crate VC-50 combo, hoping to reuse the transformers, chassis & cabinet only. All pots, jacks, (tube sockets!) are attached to the PCB, so all electronics will be replaced.
It runs (4) EL-84's and (4) 12ax7's now, and I am planning to run (4) 6V6's and (3) 12A_7's in the new amp.
The Crate is operational now (it has an intermittant problem in the PCB) and I need to know what measurements need to be taken of the PT before I disassemble/remove the PCB. It has 3 outputs which I have measured as follows:
(#1) 182 VDC / 168 VAC (no center tap) (main power)
(#2) the heaters run off a 3.2 VAC secondary (W/ center tap) (is this enough to run the heaters properly?)
(#3) there's another secondary of 12.7 VAC (W/ centertap) that I can't figure out what the Crate uses for. Looks like it goes to the effects loop(?)
The plate voltage to the EL-84's is 390 VDC.
Do these measurements sound right?
Questions~~~
***Could I run the Rectifier off of #2 (3.2 VAC) or should I use the 5v transformer that Doug sells?
***Could I step down the voltage of #3 (12.7 VAC) to 6.3 volts to run the heaters?? Am I way off base??
~These are the types of questions I need help with!!!
By the way, this is going to be my first "real" build, as opposed to just fixing/repairing an existing amp, so I'm very excited to get the parts ordered & get started!
I'm either going to build my own board using Doug's G-10 & turret lugs, or a pre-made Weber fiber board w/ eylets.
Any suggestions/help with this project would be GREATLY APPRECIATED!!!
Jeff
Edited by: j allen shaw at: 3/16/04 11:42 am
ganzonimx
Hey get your own solder
Posts: 413
(2/28/04 2:38 pm)
Re: 5F8-A Twin Project advice needed
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I'm not to sure about what I'm gona say now but I'll give it a try...
A 4 x El84 amp cathode biased produces about 30W, a 4 x 6V6 amp produces something around 50 W. So the Crate OT might be a little small for your project.
To the PT: if you have 168 VAC from your HV taps, but 390 VDC on the plates, the PS probably has a voltage doubler. Also 390V is a little high for EL84, would be ok for 6V6. To use a tube rectifier you need a center taped HV winding. With no center tap the only thing that works would be a bridge rectifier.
How did you measure the heater voltage? From tap to tap (should be 6.3 VAC) ore from tap to ground (3.15VAC).
The 12.6 VAC winding is of little use to you, you could heat the preamp tubes, but not the power tubes, but it would produce more hum.
I think that those irons would not work very well for in 4x6V6 project...
Cris
6G6
Junior tube assistant
Posts: 96
(2/28/04 3:00 pm)
Re: 5F8-A Twin Project advice needed
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Cris beat me to most of it.
The 12.7 taps would be for power to IC, which seem to like about the 17.75 VDC that you'd get once it is rectified. I doubt that those taps were ever intended to handle much current.
If you were to use a tube rectifier, you'd need to get a 5v transformer.
The PT might be OK, the OT will probably work, but it'll be working hard if it sees 50W.
An alternative to that might be to cathode bias the 6V6s and have a little less power out.
If you were to use the present power supply, you could avoid the need for a heater transformer and still have the voltage doubler.
With a GZ34 rectifier, you'd only get about 218V.
If you were to re use the EL84s, you'd have less heater current and a better match for the OT.
Of course, then it's not exactly a 5F8-A anymore, either.
Might be more of a cross between an AC 30 and a tweed Fender, which might not be a bad thing.
j allen shaw
Junior tube assistant
Posts: 6
(2/28/04 8:15 pm)
Re: 5F8-A Twin Project advice needed
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Hmm....... a "tweedy" AC30??
I think my project may be evolving! (Thanks for the help, guys!)
I actually like EL-84's, and since I already have 6V6 and 6L6 amps, let's switch gears to use (4) cathode biased EL-84's to keep the power around 30-40 watts so the OT will be happy.
So, let's start from the top:
All my measurements were taken to ground, so the heater supply is 6.3v and will be fine. (I'll just tape off the 12V supply and not use it).
For the rectifier, Doug sells a Bridge rectifier with a 3 amp/1000Piv rating. Would that work, or do I have to build one from diodes? (I see that WeberVST sells a bridge version of his "Copper Cap" rectifier that gives a 17V drop and the same "sag" as a GZ34, maybe I'll try that).
As for the "voltage doubler" on the HV, can anyone help with info on how to do that? (Does the voltage doubler come BEFORE or AFTER the rectifier, or does it TAKE THE PLACE OF the rectifier? I found this link on another post: Voltage doubler.
Is this what I need to build?)
The reason I ask about the placement of the rectifier in the circuit is that Weber will let you specify how much B+ voltage you want to drop with his zenered "Z" Copper Cap. If I have too much B+, maybe I could drop some at the rectifier).
Thanks once again for sharing your knowledge! This is something I've wanted to get into for many years, and it's great to have this forum as a resource!
Jeff
Edited by: j allen shaw at: 2/28/04 8:53 pm
HotBluePlates
I only work on Fender's
Posts: 690
(2/28/04 8:52 pm)
"Voltage Doubler"
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The power supply is not using an actual voltage doubler. It is using a simple bridge rectifier.
When you use a full wave rectifier, only one side of the power transformer is conducting at a time. When you use a bridge rectifier, both sides are conducting, so you get double the voltage output from the same transformer with center-taps, or you use a smaller winding with no center tap. That is what you have.
The bridge sold by Hoffman will certainly be sufficient for the rectifier in your amp. Or you could use a WBR from WeberVST. Your call.
6G6
Junior tube assistant
Posts: 99
(2/28/04 8:58 pm)
Re: 5F8-A Twin Project advice needed
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The voltage doubler IS the rectifier.
Another case of "there's more than one way to skin a cat".
www.ourworld.compuserve.com/homepages/g_knott/elect346.htmor if that isn't a good link, just do a search for voltage doubler.
It took me about 15 seconds to find that one.
I expect that is how it is set up on the existing amp if you have or can get a schematic for it. I do know hta crate will give you one if you need it.
What you would have is the original power supply and output sections, reproduced in point to point wiring, with an early Fender preamp section.
I have to wonder about that 390V on EL 84s, though.
I'd believe 325V.
Maybe with the choke, filter caps, and resitors between filter caps, it could come down some.
Fortunately for you , there are better minds here to help you through.
j allen shaw
Junior tube assistant
Posts: 7
(2/28/04 8:59 pm)
Re: "Voltage Doubler"
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Thanks, HotBluePlates & 6G6!!
That's the exact info I was looking for concerning the doubler/bridge rectifier! This is starting to make sense.
Regarding the plate voltage, what range should I be shooting for with the EL-84's? 325v has been mentioned, is there an optimal range? How much can they tolerate? I see 300v max on most of the tube data sheets.
Edited by: j allen shaw at: 2/28/04 9:13 pm
ganzonimx
Hey get your own solder
Posts: 414
(2/28/04 11:19 pm)
Re: "Voltage Doubler"
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I brought that up with the voltage doubler, it was just my guess that the PS could have one. I havn't seen any schematic. What do you read between the two HV taps? That would help to determine what PS design you should go for.
Cris
j allen shaw
Junior tube assistant
Posts: 8
(2/29/04 6:48 am)
Re: "Voltage Doubler"
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Chris,
I'll check that reading between the two HV taps on Tuesday, as I won't have the amp in front of me again until then.
I thank you also for your help! This is all a process of figuring out what I'm dealing with here, and your input helped to "get the ball rolling"!
Jeff
Tyrannocaster
Junior tube assistant
Posts: 48
(2/29/04 8:35 am)
EL84 plate voltages
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You usually figure 350 is tops. I have run JJ's hotter than that with no problems but I lowered the voltage as it made me nervous. I typically set them up for about 340 volts but I think 325 would be fine. 300 seems a little low for smokin' to me but it could work quite well.
Lots of posts on this subject over at the 18 watt forum (
www.18watt.com)
6G6
Junior tube assistant
Posts: 101
(2/29/04 10:11 am)
Re: EL84 plate voltages
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If you do a search for Crate amps or Saint Louis Music, you can get the tech hotline number.
Just tell them the model number of the amp
Topic: 5F8-A Twin Project - Flabby/Farty Bass? &nb (Read 3803 times)