Fisher PT capacity & what rectifier is this?

[Tyrannocaster]

I'm trying to figure out if the PT from a Fisher X-100 (which uses 4 7189s and 5 12AX7s) can handle a pair of 6L6s. (I'm pretty sure it would if they were cathode biased.) The voltage looks about right for a Tweed Pro or something like that on the X-100 schematic, but the rectifier they are using isn't like what I see on other amps. There's no center tap, but they are only using two diodes, which confuses me. Here's the schem:



If I use a regular four diode bridge like I'm used to with this PT what am I likely to get?

Excuse the beginner question, but I haven't built an amp in a long time and I'm having to relearn a lot.

[Tyrannocaster]

From lots of Googling I'm figuring this is a voltage doubler; I've never used one of those before. I guess I would have to if I wanted to use this PT, though, unless I wanted really, really low voltages.  Are there any pitfalls here for the unwary?

[jjasilli]

Today should henceforth be known as voltage doubler day!  See the nearby thread by Colas.  

Yes, this is another full-wave voltage doubler circuit.  Yes you can covert it to bridge rectification, but it will have 1/2 the voltage (with 2X current handling capacity).  Unfortunately, 1/2 of 390 plate volts is only 195V which is low for the intended power tubes.  Tube charts list 250V as the lowest for 6L6.

To calculate power handling capacity, go to the tube charts.  E.g. Google: Franks Tubes.  Look-up what current all the power tubes draw at full signal, both plate & screen.  Small bottle tubes draw so little mA, that their current draw can be overlooked.  At 400V, a pair of 7189's draws 130mA, so 4 tubes draw 260mA.  At 450V 2X 6L6's draw 232mA, or less depending upon bias and actual plate voltage.  Looks good to go, keeping the doubler circuit.

But why kill the stereo amp?

[Tyrannocaster]

Today should henceforth be known as voltage doubler day!  See the nearby thread by Colas.  

Yes, this is another full-wave voltage doubler circuit.  Yes you can covert it to bridge rectification, but it will have 1/2 the voltage (with 2X current handling capacity).  Unfortunately, 1/2 of 390 plate volts is only 195V which is low for the intended power tubes.  Tube charts list 250V as the lowest for 6L6.

To calculate power handling capacity, go to the tube charts.  E.g. Google: Franks Tubes.  Look-up what current all the power tubes draw at full signal, both plate & screen.  Small bottle tubes draw so little mA, that their current draw can be overlooked.  At 400V, a pair of 7189's draws 130mA, so 4 tubes draw 260mA.  At 450V 2X 6L6's draw 232mA, or less depending upon bias and actual plate voltage.  Looks good to go, keeping the doubler circuit.

But why kill the stereo amp?

Normally I would agree with that last question but it has a major problem with its input selector switch and a replacement is just impossible to find, since it was a proprietary part. The trannys are good, though.

Thanks for that info. I thought that if I used a normal bridge rectifier circuit like the ones I am sort of familiar with I would get voltages of (roughly) 273 volts (195 x 1.4), or a usable (if a bit low, depending on the design you are looking at) voltage for EL84s. The 7189s have similar data but can live with much higher voltages. So with the same OT I could use two 6L6s or 4 EL84s and an 8 ohm load. But if you are right I would only have 195 volts, correct? Then I have to either use the 6L6s or the 7189s because I'm uncomfortable with that voltage on EL84s. But the 7189s aren't available except as NOS and they have become expensive. I think the Fender idea is the right one for this, with the OTs going to other projects (or course, since there are two of them). Man, rectification is confusing! I just want to figure out what my options are here.

[sluckey]

What's wrong with using the power supply as it is? It gives you 390Vdc with enough juice for 4 7189s. Should do what you want.

But, measure the ac voltage between the R-Y lead and the Red lead. Multiply that reading by 1.414 and the results will be the unloaded B+ available from a full wave bridge.

[RicharD]

I'm with Sluckey.  Voltage doublers are cool.  Yes you have a current trade off and there is a DC bias on the secondary, but Fisher made relatively high end stuff and they weren't concerned.  It is full wave rectification so it's clean.  You just gotta watch your load as you would with any PSU and yer fine.  That tranny with a voltage doubler will spit out 200mA at 390V all day long.

[Tyrannocaster]

Absolutely nothing wrong with using the power supply as it is, and that's what I will end up doing, I'm sure. I was trying to fit this thing into the set of known objects that use rectifier circuits I'm familiar with and get a handle on it, that's all. But I won't use the 7189s with it - they stopped making them in 1973 and I don't want to have to chase down expensive forty year old tubes when they need to be replaced. I'd much rather use them in other projects that use lower voltages (EL84) where they will last forever. That's why the lower voltage from the conventional bridge was interesting, but using 6L6s in a Tweed project sounds perfect for this PT. And with two OTs on hand from the same amp, I'm set for some EL84 amps, too.

Thank you all for your replies; I keep a scrapbook with this stuff in it.

[jjasilli]

it has a major problem with its input selector switch and a replacement is just impossible to find

For today's use, maybe only one or two inputs would be fine.  A single input could be hardwired, say for a CD player or iPhone.  A toggle or rotary SW could be used if desired for a second input.

I use vintage tube amps for home theater systems.  Either tube power amps, or integrated tube amps with their preamps bypassed.  The tube amps are fed into one hardwired input, from the SS home theater receiver, which does all the input selection, On/Off, & vol control -- by remote control.  (This is essential for WAF -- Wife Acceptance Factor -- otherwise no tube amps!)  

[sluckey]

I do not see this PT as a voltage doubler.

The PT ain't a voltage doubler, but the rectifier circuit certainly is.

I also see a lot more capacitors used prior to providing the B+.

Part of the nature of a voltage doubler. Maybe google voltage doubler to get a handle on this.

I would make a bet the 7189s are heated in series.

You lose. Did you even look at the schematic?

I am wondering what V-3 is, since it has about 25-30v on its heater.

That 30 volts is the cathode voltage of four 7189s. The cathode resistance for those four 7189s  consists of a 22Ω resistor in series with V1, V2, and V3 filaments. The 30 volts divides across those four components. There is not 30 volts across any one tube filament. Just having 30v one one side of a tube filament is meaningless. You need to know the voltage ACROSS the filament pins. Heck, I've seen many 5U4s that had 400v on its heater!

[Willabe]

I do not see this PT as a voltage doubler.

Look at where the bottom leg/lead from the PT B+ secondary is wired to.

The bottom leg/lead from the PT B+ secondary is _stacked_ on top of the bottom B+ filter cap, in-stead of going directly to ground, which lifts the B+ voltage _above_ ground at that point at the top of the bottom filter cap/bottom of the top filter cap.

This is also why when a volatge doubler is used it cuts the B+ current in half.

Half of the PT secondary B+ current goes to fill/re-charge the bottom filter cap and the rest of the PT's secondary B+ current goes to the top filter cap to fill/re-charge it.


                               Brad          

[Tyrannocaster]

Funny how once you see something you start coming across it everywhere; I started making Doug's stereo preamp while I'm waiting for all the parts for my next amp to get here. I am using an old Heathkit table radio for the power supply and the rectifier is...you guessed it...a voltage TRIPLER. Had to look that one up too, as it isn't exactly like the doubler but it's similar enough that at least I wasn't gobsmacked by it. 

[jjasilli]

I do not see this PT as a voltage doubler.

Look at where the bottom leg/lead from the PT B+ secondary is wired to.

The bottom leg/lead from the PT B+ secondary is _stacked_ on top of the bottom B+ filter cap, in-stead of going directly to ground, which lifts the B+ voltage _above_ ground at that point at the top of the bottom filter cap/bottom of the top filter cap.

This is also why when a volatge doubler is used it cuts the B+ current in half.

Half of the PT secondary B+ current goes to fill/re-charge the bottom filter cap and the rest of the PT's secondary B+ current goes to the top filter cap to fill/re-charge it.
Brad          

I think the confusion here is incorrect focus on the PT by drgonzonm.  The tranny performs a multiplication operation upon the voltage input.  If the the PT takes a 120VAC input and puts out 240VAC, then drgonzonm considers the PT to be a "voltage doubler". Since this PT puts out some other voltage, then the PT  is not a "voltage doubler".  But this is wrong nomenclature.  A "voltage doubler" refers not to the PT, but rather to the type of rectification/filter circuit which follows the PT. 

[PRR]

I still say: keep it minimally-modified. Replace V4 V5 with 12AU7. Strap the two power-amp inputs and outputs together. Input at R31 R32, output becomes 4 and 8 ohms. Build a one-12AX7 preamp and tone-stack to drive that. You will have to calculate cathode resistors to replace the V1 V2 V3 DC-heat scheme. Run EL84: modern production WILL take it, the old 300V rating no longer applies. Also there's only 360V across the tubes. And EL84 are too cheap to care. C23-C26 could be 0.01uFd to reduce bass-fart. R51-R54 could be 10K each or so for less-abrupt overload. _I_ would rip out R41 R42 R72 R73 for simple robust 56K (330K||68K) resistors (the trim is for THD-meter happiness, not sound; and when the trimmer fails it will suck).

[DummyLoad]

rather clever way to fix bias with positive only power supplies. raise the cathodes to +30V and apply +18 to g1. result (-)12V bias. works as long as Vg1 is < Vk, correct? since the cathodes are at +30V and the plates are at +360V then Vtube is +360V, ya, i kno, being capt. obvious...but just thinking out loud. :-)

--DL 

[PRR]

> clever way to fix bias

It's self-bias. Just a big resistor in the cathode return.

Yes, the resistor drop is 30/12 times what the power tubes need, but because the G1 is tapped at a divider that makes no difference.

The side effect is DC heat for the critical Phono stage and another stage.

[DummyLoad]

It's self-bias.

i'm a dummy- didn't see that. now i.c. & got it. thnx.  :-)

[TIMBO]

So if i have a PT with a secondary of 230v (no CT) and use this circuit i could get an unloaded output of approx of 460v???

[DummyLoad]

So if i have a PT with a secondary of 230v (no CT) and use this circuit i could get an unloaded output of approx of 460v???

actually around 600V.

--DL

[TIMBO]

WOW, A bit tooooo much for a pair of 6l6s??

[PRR]

> secondary of 230v

I got about 560V loaded.

The "doubler" really doubles the PEAK voltage. Peak of 230V is 230*1.414 = 325V. Double that is 650V. Unless the PT is grossly oversize, it will sag, and sag is doubled too. So easily 600V under small or medium load, 560V when I was abusing a 6550.

While that is not kosher on 6L6, it is legal-enuff on 807, and groovy for a pair of 6550/KT88 working 100 Watts into 5KCT.

[TIMBO]

Thanks PRR