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spitfire questions

7/9/1998 9:16 PM
Mark Buckingham
spitfire questions
After I got a great deal on parts, (got the power and output transformers from a good friend for free), I've decided to build a spitfire clone. All I have left to obtain is a chassis to put it in and start soldering.  
So here's a couple of questions-  
1. The power transformer I got puts out 326-0-326 vac. I need to know if I'm going to have too much power here, as the spitfire schematic shows a 285-0-285 vac secondary.  
2. I'm planning to use 2 high speed diodes to rectify the voltage. The book I borrowed from the library says that the average value of the dc voltage is .637 * the ac voltage. This would give me 415 vdc before the first filter cap. (I think.) Is this the right way to determine this?  
3. I'm planning to wire in a pentode/triode switch similar to the one on Duncan's web sight for the blues-112. In this, he uses 22k resistors to tie the screens to the plates. Should this work ok here also?  
4. The master volume circuit looks a bit odd. Does this work well, or should I do the stereo volume control trick instead?  
5. If the triode switch works, I'm guessing I'll get about 3 watts out of this thing in triode mode. To me, that means I can get power tube distortion much earlier. Right?  
Thanks for any help anyone can give. My main goal is to build something that I know already works so I can have a nice circuit to add to or change. (And having yet another guitar amp is very cool too :) )  
7/9/1998 9:30 PM
Mark Buckingham

One more question-  
I've got a nice 475 vdc, 20/20/20/20 uf multisection cap, as well as a 450 vdc 47 uf cap. My friend (not knowing the exact power the transformer put out), suggested that for the first cap posistion in the power supply, to put one of the 20 uf sections in parallel with the 47 uf cap, then use the other 20 uf sections normally further down the line. Will this work ok? I really don't want to re-buy the caps if I don't need to. Thanks.
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7/9/1998 11:17 PM

the general rule of thumb.....mine, is to take one side of the center tap trannie and multiply that votlage by 1.4142.  
So, 326 x 1.4142 = 467vdc when using a capacitor filter.  
This will be a little less then the actual unloaded power supply B+.  
When you load the supply down, you'll notice a pretty good voltage drop. But not 100v.  
You can do it the way you did, (.637 x PPvac= 415.3vdc) but, that is not the voltage you will measure when you hook it up to a capacitor.  
There is a diff between average, RMS and peak voltage.  
The 415vdc is what you might see when the power supply is fully loaded but, with only a single pair of EL84s at about 45ma each, I bet the voltage will still be a bit high for a safe running class A amp with the two EL84s. the preamp tubes are an extremely light load by comparison.  
Since it is a center tap transformer, you could install the dreaded 30v to 50v/ 50 watt ZENER diode to drop the over all voltage.  
I don't know why you are using high speed switching diodes at 60Hz for your power supply but make sure that the combined voltage rating of diodes is twice what the supply can peak at.  
Two or three 1000v/1amp 1N4007s in each leg will probably run for ever though and you can buy them from 6 cents to a quarter each.  
Duncan is such a cool guy, all you need to do is write him an e-mail message to ask him these same questions and he'll write you a novel about his amps and such.  
The master volume on the Matchless works OK but it is a slightly different sounding master volume then what most people expect to hear.  
Steve A. and I have been discussing this MV thing for a few weeks off the website here.  
My thoughts are that the dual gang pot thing works pretty good when you block both the inputs and the wipers of the pots with a .047uF to .1uF cap and then tie the audio low ends together and ground them. The caps from the wipers now go onto the grid load resistor and grid stopper resistor of the final tubes. You can get solid preamp distortion this way, if that's what you're after.  
Fixed or self biased asside, this makes it so the MV doesn't change any DC bias or DC grid load of the final stage but most assuredly sends all the signal you need on to the final or right to ground.  
We should get a thread going about the merits of all 4 or 5 types of MV controls.  
7/10/1998 9:36 PM
Mark Buckingham

Thanks, Bruce.  
Could you tell me how the zener idea works? Do I install it between the center tap and ground? (And now for a probably silly question) Would a 50v zener drop the overall voltage 50 or 100 volts dc?  
I think I'm going with the dual gang pot for the master volume. What value & wattage pot do I need?  
Thanks again.
7/11/1998 8:48 AM

Yes, the 50v zener will drop your overall B+ by about 48-49 volts.  
The 50 watt Zener mod works pretty good but they are'nt cheap.  
Plus, the zener uses valuable current to drop the voltage and dropping 50v at over 100ma will eat abt 5 watts of power from your power transformer.  
For the little amps (under 20 watts) I don't use the chassis stud mounted 50watt zeners.  
They cost way too much..... about 25% to 35% of the cost of a new small VA power transformer!  
Instead, use a couple lower voltage... 5 watt zeners and put them in series. About $1.00 each.  
The voltage drop is now divided across two 5 watt zeners for a total of 10 watts for around $2.00.  
I've been using this combination in small amps for a while and have never had a failure.  
Yes, the zeners go between the center tap and the chassis ground or wherever you grounded the center tap.  
If you need to drop of 30v, use a pair of 15v-16v zeners and this will be pretty close.  
Each zener diode will drop 15v-16v but the max current draw will through the power supply will pass through each.  
So 15v x 100ma = 1.5 watts. But since you used two in series each only has to handle 1.5 watts but the total voltage drop will be closer to 30v.  
My zener diode parts trays have an assortment of 12v through 30V at 5 watts aI always seemtobe able to come up with a pair of equal valued zeners that can drop the B+ to where I want.  
Tech America, 1-800-877-0072. sells them for around $1.00 or so each,  
The dual gang pot I use is a generic ALPHA or MOUSER 1/2 watt dual gang audio pot at 1M.  
Dan Torres uses a 500K in his kits.  
Either are OK and the 500K acts like it is doing more with the rotation of the pot since the last 250K or more doesn't; seem to effect the sound much.  
At one time I had a bunch of MOUSER dual ganged 1M A pots, so I just try and use them.  
7/11/1998 7:05 PM
Mark Buckingham

Bruce, thanks for all of your help. I've got one more question about the zener thing...  
Does the zener also handle the rectification, or do I still put the diodes on the other two legs of the transformer?  
7/12/1998 2:53 AM

Still use the regular diodes in a normal full wave rectifier layout.  
All your doing is changing where the zero vac point is on the windings relative to the ends where the solid state rectifier will be. Yes this will work with a tube rectifier if needed.  
Here's some more of my hammer mechanic backyard all night TV repair shop mentality ....  
And please, don't treat this as Gospel, it's just my loose idea behind this madness...  
The center tap of your power transformer is in the center of the secondary windings so the voltages swing up and down from that point like a teeter-toter,.... if the CT is grounded.  
If you hold the CT up from ground with the zener,then the reference point of ground is shifted by the value of the zener diode.  
But, even though it does take a current flow through the device to avalanche it, the overall voltage across that zener is only as high as it takes to overcome it's reverse voltage breakdown point and avalanche the diode to "turn it on", but as the voltage drops, (it's AC), it shuts back off.  
I guess since it is AC, it must turn on and off at 60Hz and the end results seems like a steady drop of the rated zener voltage.  
I'm sure someone has a better explaination then that! Help.  
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