Tube Amps / Music Electronics
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|6/4/2005 4:24 AM|
||California Dreamer question|
Hello friends! I've decided to make the California Dreamer amp, as in the ax84 website, following the lite schematics you can find here http://220.127.116.11/ax84/media/ax84_m143.jpg
On another schematics (http://18.104.22.168/ax84/media/ax84_m274.pdf) it seems that this amps has about 400V on the plates of the 6V6, and the suggested transformer for using a 5u4GT rectifier is an Hammond 272DX. But... isn't it a 300-0-300V tranny (from the Hammond website)? It's a bit low to get 400VDC... and on that schematics near the PT hv windings you can read 328VRMS... ??? Something's wrong. I just need to know the proper HV voltage because I must have my PT custom made, as here in Italy Hammonds are extremely expensive.
Thanks to all in advance!!!
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|6/4/2005 4:28 AM|
And, let me ask please, if someone built an amp like this and if it's worth the effort for getting that vintage fender clean and smooth breakup...
Or if there are any mods you suggest me to improve the tone of the amp...
|6/4/2005 7:47 AM|
For 400VDC @ 200mA using a 5U4, I would suggest a power transformer with either a center-tapped 640VAC @ 240mA winding, or (preferably) a 320VAC @ 360mA winding for a full-wave bridge rectifier (you can still use the rectifier tube if you want). I would also specify 5VAC @ 4A for your rectifier heater winding, and 6.1VAC @ 4A for your tube heaters. This will be a 160VA transformer and bigger/more expensive than a stock Fender Deluxe transformer, but it will run cool, last forever, and give you a quiet, reliable amp.
The circuits you posted look very similar to a Fender Deluxe and Deluxe Reverb amp, so yes, they should give you that Fender sound. If you are having an output transformer custom-wound you might want to add 40% ultralinear taps, which should give your output stage a smoother break-up than pentode operation along with a slightly cleaner clean sound.
|6/4/2005 7:53 AM|
Thank you! So if I want to use a standart full wave rectifier (but I will protect the rectifier tube with ss diodes) i have to use a 320-0-320 transformer or near? So the Hammond 272DX is clearly wrong....
|6/4/2005 8:27 AM|
And I'd like to know about the OT... I can get an Hammond 1608 for 55euro, a taiwanese Fender Deluxe Reverb replacement for 26euro, a not-taiwanese one for 58euro, an Hammond 125E for 30euro. Prices here are a bit high... What should I get? I would do for the taiwanese one to save some money, but it makes me think about it has only the 8ohm output (instead hammonds have more impedance options), and maybe taiwanese construction and materials will lead to a fizzy and bass-lacking transformer... And how about the Hammond 125E?
I know that for the best sound I'd have to buy the 1608... it has the best bass.
What do you suggest me?
|6/4/2005 9:04 AM|
If the 272DX puts out 300-0-300VAC it should give you about 380VDC before the rectifier, and by all accounts Hammond transformers put out higher than rated voltage, at least in the U.S. I don't know if this makes it "clearly wrong" or not; 6V6's will be quite happy at B+ voltages between 300 to 400V.
I don't know how any of those OT's you mentioned sound, but I would not use a 125E if you are looking for the most bass response. I have installed a 1645 in a Deluxe circuit that worked very well, and I would personally use a 1650H; both of these should have better bass response than the 10-watt 1608. I realize these OT's may be too expensive, but maybe they can serve as a guide for primary impedance and wattage rating for you.
|6/4/2005 11:07 AM|
Giame, be careful about the transformer voltage needed for a full wave bridge.
A tube rectifier normally uses a centretapped secondary feeding a 5U4 or 5Y3 type tube that has 2 internal tube diodes. In this circuit the centretap needs to be grounded so each half of the HV secondary must supply the whole voltage for 1/2 of the AC cycle. So a transformer with 300 volts either side of centretap (300-0-300vac) will deliver roughly 300 volts out of the rectifier to the filter caps, less tube losses. The caps will charge to the peak of the AC wave and we'll get 1.41 x 300= 423 vdc peak. We'll lose at least 23 volts in the rectifier tube and the idle currents of the output tubes will pull it down even more.
A full wave bridge does not normally use the centretap. So both sides of the secondary hit the recifiers. That 300-0-300 vac Hammond now looks like 600 vac to the rectifier bridge! SS diodes lose only a bit over 1 volt so you can see that by the time we get to the filter caps we are now likely to see nearly 900 volts!
Things can still be made to work - just make sure you ground that centretap even with the fullwave bridge. That will make 2 of the 4 diodes non-functional and the circuit will behave the same as with a tube rectifier.
If you don't have a centretap you can't use the usual tube rectifier.
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