The Fisher 500C/800C Challenge (long..dialup warning)

[Tom Bavis]

The DC filaments are 22V @ 300 mA. 4-12V tubes in series-parallel, or 4-6V tubes in series. Since the 12aX7 can run either way, 2 6922s and two 12AX7 could be run in series too.

As for high line voltage, a pair of Keystone inrush limiters on the primary will drop a few volts. The run HOT, keep 'em away from other parts and wiring.

[Eli Duttman]

All,

Tom B. is correct. The DC heater supply is "25" V./300 mA. My focus was on 2X series pairs in parallel and that's too narrow.

OK, 4X 6.3 V./300 mA. bottles in series work. A 12AX7 and a 6922 for the phono section combined with the 2 splitter/driver tubes looks good. FWIW, I'd still prefer that a UCC88 be combined with a 7058, as both twin triodes have internal shields that could aid the channel separation cause. Both tubes are in both channels.

The ECC808/6KX8 is interesting, but the TFK data sheet shows a 340 mA. heater draw.

[valveitude]

Hello Tom,

The DC filaments are 22V @ 300 mA. 4-12V tubes in series-parallel, or 4-6V tubes in series. Since the 12aX7 can run either way, 2 6922s and two 12AX7 could be run in series too.

You are correct sir..thanx for pointing that out . I am still chewing on the reg'ed 6v line possibility though..but you've pointed out a fallback position for me if it becomes too impractical to implement the seperate 6v route.

My thinking is this..

1) If I can re-alocate the bulb current to run BOTH tubes of the phono section, I'll be able to use the 4 dc "slots" in the original design for the remaining line tubes, eliminating all line level ac filaments.

2) Potentially more importantly, I understand (i think) that elevating the filament helps reduce noise (helps prevent the heater from acting like a cathode..right ?). This is most critical in the phono stage. Not only can I not do this with the stock supply,
the reverse is true..the first tube in the string is -22v from ground, and the second is -12V.

Eli,

Thanx for continuing to bat this around with me.

When cap. I/P filtration is used, the rectifier conducts in brief, high current, pulses. The heating in the power trafo is I^2R averaged over time. A DC draw 1/2 the AC RMS capability is SAFE, from a heating perspective. Please observe that heating increases as the value of the filter cap. increases

Got it.

Schottky diodes are best and the total drop is about 1 V. You'll get approx. 7.5 VDC at the filter cap.

(6.3*1.414)*109%(increase from 110v nominal)=9.7 9.7-1(schotky Vd)=8.7 8.7-6.3=2.4 max reg. dropout.

This is a slim margin to be sure, but I may be able to find a reg. that will work....any suggestions.

If I can get the reg'ed 6v worked out, I'm thinking I can run a second bridge off the bias supply to give me a +24v filament supply bias.

As for the current, the dial lamps I've ran into are 3W, on a 6v supply this works out to around 500mA each, times three that gives me 1.5A..with your recommended safety margin, I can run 2 300mA tubes.

What have i missed...will this work ?

Thanx,
Casey

[Eli Duttman]

(6.3*1.414)*109%(increase from 110v nominal)=9.7 9.7-1(schotky Vd)=8.7 8.7-6.3=2.4 max reg. dropout.

No sir. 6.3 * 1.09 = 6.87 6.87 - 1 = 5.87 5.87 * root_2 = 8.3 You are OK with 6 VDC on the heaters. That leaves a THIN margin of 2.3 V. With a low dropout regulator you could be OK.

There is (IMO) a better way to skin this cat. Use 2X Schottky diodes as a "full wave" doubler. Use 2X 1000 muF./15 WVDC 'lytics in the doubler stack. Now, a low cost 7812 regulator IC has enough headroom to function well. Wire 2X 6.3 V/300 mA. heaters in series or 2X 12.6 V./150 mA heaters in parallel. Notice that the energy stays the same, but it's distributed differently. BTW, if you use dropping resistors to control the AC heater voltage, connect the doubler directly to the filament winding. Extra headroom for the regulator IC CAN'T hurt.

I still prefer a 7058/UCC88 combo for the phono stage (internal shields), but by using LEDs for illumination, you do get to put DC on 2 more heaters.

[poobah]

Hello Eli,

Would you take a look at a threead called "simple way to measure source resistance?" I think I gave some one a bum steer...

[valveitude]

No sir. 6.3 * 1.09 = 6.87 6.87 - 1 = 5.87 5.87 * root_2 = 8.3 You are OK with 6 VDC on the heaters. That leaves a THIN margin of 2.3 V. With a low dropout regulator you could be OK.

Oopsies...seems I did things in the wrong order...thanx for the correction

There is (IMO) a better way to skin this cat. Use 2X Schottky diodes as a "full wave" doubler. Use 2X 1000 muF./15 WVDC 'lytics in the doubler stack. Now, a low cost 7812 regulator IC has enough headroom to function well. Wire 2X 6.3 V/300 mA. heaters in series or 2X 12.6 V./150 mA heaters in parallel. Notice that the energy stays the same, but it's distributed differently. BTW, if you use dropping resistors to control the AC heater voltage, connect the doubler directly to the filament winding. Extra headroom for the regulator IC CAN'T hurt.

I like it..thanx again. On the filament V drop resistors for the AC bottles, do I want to put a resistor on each leg to keep it balanced, or does it matter ?

I still prefer a 7058/UCC88 combo for the phono stage (internal shields), but by using LEDs for illumination, you do get to put DC on 2 more heaters.

It's a question of sourcing for me , the market is flush with ECC83/ECC88 variants. Its bad enough with only 2 sources of 7591's. I'll check out the 7058/UCC88 source options though, the internal shields are appealing.

Onto the line/tone stage. (I'll be re-posting some of the schematic from the Sams..much clearer)...



...Do I really need 2 stages (I/P O/P) to drive the the Baxendahl ? One of the appealing features of Thorsten's phono stage is the fact it can drive a low load (10k according to Thorsten...you out there Kuei ? I'd love to get your take on this), this would allow me to shunt input to the tone circuit with a suitably low (47k?) resistor, giving the phono stage a stable load. That seems to me would eliminate the need for the first stage, I assume the second stage is to offset insertion loss (I'm guessing here). If this is the case, it would free up a triode section. I would then be able to run two 12ax7 sections in parallel for the 1st stage of the phono, dividing my noise by 2 in this critical stage( it would however, put the second phono stage back on the negatively biased filament supply..oh well, this is a world of compomises, isn't it). The second stage would only be in line when the tone is switched in, switched out, the amp is following the rule of 2's. Two phono stages, and 2 PA stages (one Schmidt splitter/VA stage and the outputs).

This is starting to look good (in my head anyway).

Thoughts?

Casey

[Eli Duttman]

Casey,

Except for the heater (12.6 vs. 6.3), the UCC88 and the ECC88 are the same tube. Both have an internal shield connected to pin 9. Some scrounging will be required to come up with UCC88s.

The 7058 is a different story. GE built 7058s are under $10 from AES. RCA built tubes will cost more.

RES lists a price of $46 for a 6KX8. That will be either a TFK or a Mullard. Getting 1 of those into the phono section is worth a struggle. Perhaps a LT brand low dropout regulator IC would be OK with the limited headroom available. Check out the data sheets. You'd need 640 mA. at 6 V. for a 6KX8/6922 combo in the phono stage.

Parallel 'X7 sections forces a recalculation of the RIAA network. NYET!

DC on the phono section heaters is plenty quiet. IMO, biasing off B+ is not necessary. Also, you have less ground watching to do.

[valveitude]

Eli,

Except for the heater (12.6 vs. 6.3), the UCC88 and the ECC88 are the same tube.

Since the filament supply is flushed out, it doesn't seem to make a difference then.

RES lists a price of $46 for a 6KX8. That will be either a TFK or a Mullard. Getting 1 of those into the phono section is worth a struggle

So, the 6KX8 is a special 12AX7 then, could you eleborate on it ?
I'm assuming since its a 6V tube, the pinout is the same as a 12AX7 wired for a parallel filament...true?

You'd need 640 mA. at 6 V. for a 6KX8/6922 combo in the phono stage.

Or 12 V @ 320Ma

[Eli Duttman]

Casey,

The triodes in the 6KX8 are electrically equivalent to those in the 12AX7. However the 6KX8 is "6" V. only and it doesn't pin out the same as the 'X7. The key feature of the 6KX8 is its internal shield. 6KX8/ECC808 Data Sheet

[valveitude]

Eli,

The triodes in the 6KX8 are electrically equivalent to those in the 12AX7. However the 6KX8 is "6" V. only and it doesn't pin out the same as the 'X7. The key feature of the 6KX8 is its internal shield. 6KX8/ECC808 Data Sheet

Thanx for the datasheet, I couldn't find one. Nice toob!!

So what to do with my NOS Telefunken smooth plates ?

Would a paralleled 12AX7 work out for the Schmidt circuit, it would have twice the gm of a single section, and provide more gain than the 12AT7 ?

Casey