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Old 15th June 2002, 04:56 AM   #1
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Default 807 pp amp - help

Hi all,
I am planning to build the 807 ultralinear PP amp as described in Mr. J. C. Morrison's Fi Primer (http://tinpan.fortunecity.com/saints...mer/807-pp.gif)

I have some doubts, and would be very thankful if anybody could clear those.

1. What is the power transformer rating? Is it:
Primary : 230V AC (local supply in my area is 230 V AC)
Secondary : 0-220V @ 500 mA (for B+)
0-6.3V @ 5A (for filaments)
0-100V @ 500mA (for -100V)

2.Can I have multiple secondary windings in the same transformer for B+, filaments and -100V supply, or should these be seperate transformers?

3. From the anode of 6SL7, after the 68K 1W resistor, and the 5K 1W resistor, an arrow is shown. Is this to be connected to B+?

4. In the schematic, just before the 807s, I see a resistor network comprising of a 15K, two 5K Lin pots in parallel, and a 1.1 K resistor connected between B+ (I think) and ground. What is the purpose of this? What is the setting for the 5K linear pot and how is it arrived at?

5. In the schematic, the B+ is shown as 230V, but if I use a power transformer with 220V Ac secondary, use full wave rectification as shown, and use a 800uF capacitor for filtering, won't the DC output be somewhere around 311 V?


Thanks in advance for any assistance.
regards,
rk
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Old 15th June 2002, 06:42 AM   #2
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Hi RK,

I've never seen this amp before, but I'm partial to 807's myself.

<b>1. What is the power transformer rating? Is it:
Primary : 230V AC (local supply in my area is 230 V AC)
Secondary : 0-220V @ 500 mA (for B+)
0-6.3V @ 5A (for filaments)
0-100V @ 500mA (for -100V) </b>

Primary: 230V as that's what's local for you.
Secondaries: I'm assuming that both channels are fed from the same supply.
- B+: 0-220V @ 500mA. This is nearly what the circuit is drawing at idle. I would have a minimum of 1.5 - 2 x this myself.
- filaments: 5A @ 6.3V looks a bit marginal to me. Each 807 draws 0.9A, each 6SL7 draws 0.3A, so that comes to (4x0.9+3x0.3= 4.5A). Personally I prefer to overrate and run things like transformers cooler, so I'd have a 6-8A winding at least.
- 0-100V@100mA max and even that's way overkill. Each 6SL7 will only be drawing a couple of mA each (1 -1.5 ea at idle), so it needs to sink 4 x 3 mA roughly. The bias network for the 807s is drawing roughly 5.5mA

<b>2.Can I have multiple secondary windings in the same transformer for B+, filaments and -100V supply, or should these be seperate transformers?</b>

Yes, you can have all the windings on the same transformer core, so long as it is specified for the total current draw and expected temperature rise.
If you are going to get the power transformer custom made, then it might be cheaper to get a 230-230 isolation transformer, plus others for the filaments and -100V bias, buying off-the -shelf items. If you are getting one wound, spend another couple of dollars and get an electrostatic shield installed. It will reduce a lot of the mains induced noise in the amp. I like EI cores for the same reason.

<b>3. From the anode of 6SL7, after the 68K 1W resistor, and the 5K 1W resistor, an arrow is shown. Is this to be connected to B+? </b>

Yes.

<b>4. In the schematic, just before the 807s, I see a resistor network comprising of a 15K, two 5K Lin pots in parallel, and a 1.1 K resistor connected between B+ (I think) and ground. What is the purpose of this? What is the setting for the 5K linear pot and how is it arrived at?</b>

This network connects to the -100V rail, and is a resistive divider network to set the bias for the 807s. This is called fixed bias. This network will adjust between -5.2V and -28.9V. I don't know what level of bias JCM wanted on the tubes, but I'd guess somewhere in the middle of the range, say 18-20V. Do you have any other information as to the correct bias voltage?

<b>5. In the schematic, the B+ is shown as 230V, but if I use a power transformer with 220V Ac secondary, use full wave rectification as shown, and use a 800uF capacitor for filtering, won't the DC output be somewhere around 311 V?</b>

Theoretically yes, you'll get 311V at the top of the first cap. Throwing some numbers into quickly PSUD, I come up with between 290 and 255V, because you'll have a voltage drop across the 10 ohm resistor, dependent upon the current draw of the circuit.
==========================================
After I'd typed all the stuff above, I found in my datasheets for the 807 a set of specs for class A tetrode operation, that are quite similar to the circuit JCM has designed.

Plate V: 250V
Screen V: 250V
Plate current (no signal): 120mA/tube
Plate current (max sig): 140mA/tube
Load plate-to-plate: 5k
Grid voltage: -16V (answer to Q4)
Power output: 14.5W
THD: 2% (remember this is tetrode, not ultra-linear, and there is no NFB applied)

I also found the description of the 807PP amp on the same site you linked the schematic from: 20W @0.1% UL. The datasheet specs just above look pretty close.

Sorry my response was so disjointed, but I hoped it helped some anyway. I'll look at it further, and if you have any more questions, I'll do my best to answer them.

Cheers
Brett
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Old 15th June 2002, 08:22 AM   #3
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I have quite a few 807s -- this amp looks interesting.

What is the rationale behind cross-connecting the cathodes of the 807 to the secondary of the OPT?

dave
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Old 15th June 2002, 12:48 PM   #4
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The connection of the 807 cathodes to the OPT is for negative feedback. The 8 ohm tapping is the centre tap of the secondary. If the voltage feed back to the cathode is in phase with the grid voltage then the change in grid-to-cathode voltage is reduced, therefore negative feedback.

Actually, now I think about it, the impedance ratio is the turns ratio squared, so is the 8 ohm tapping actually the centre tap of the complete 16 ohm secondary? What I mean is this: Lets say we had this transformer and it had an 8 ohm secondary winding and it had 8 turns on it's secondary. We then decided we wanted it to be a 16 ohm winding instead. So 8 turns squared = 64. Double that = 128. Square root of 128 = ~11.31 turns. So we have 8 turns and 3.31 turns either side of the 8 ohm tapping. It may be the impedance centre tap but it is not the voltage ct. SO if we take negative feedback from this arrangement then it will be pretty lopsided!
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Old 15th June 2002, 07:01 PM   #5
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Quote:
Originally posted by Circlotron
[B]The connection of the 807 cathodes to the OPT is for negative feedback

<snip>

negative feedback from this arrangement then it will be pretty lopsided!
Thanx... a conversion to cathode bias would be a consideration.

dave
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Old 15th June 2002, 07:36 PM   #6
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Lightbulb Correction

I just realised I made a mistake in the plate current values in my first post. It should be
Plate current (no signal): 120mA/tube pair
Plate current (max sig): 140mA/tube pair

I'll run the simulation again in the morning and see what the transformer secondary voltage for the B+ supply should be with the lower current draw. 500mA should be enough for the B+ winding now. Sorry for the confusion.

Dave (planet10) said

<b>a conversion to cathode bias would be a consideration.</b>

I've never been a big fan of fixed bias myself. I wondered whether you'd seen this old Sound Practices article on it.

http://www.southernelectricaudio.com/images/fixbias.pdf

There's also a great and very detailed STC datasheet for the 807 here. Best I've seen by a long way, but 5.5M in size.

http://www.retrovox.com.au/STC807.pdf
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Old 15th June 2002, 07:42 PM   #7
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Default Re: Correction

Quote:
Originally posted by Brett
I've never been a big fan of fixed bias myself. I wondered whether you'd seen this old Sound Practices article on it.

http://www.southernelectricaudio.com/images/fixbias.pdf
I tended toward cathode bias based on Allen Wright's preferences, but the above article really nailed it for me.

Quote:
There's also a great and very detailed STC datasheet for the 807 here. Best I've seen by a long way, but 5.5M in size.
I got scans of this same datasheet from a fellow in Perth a while back.

dave
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Old 17th June 2002, 05:17 AM   #8
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Hi all,
Thanks for all the feedback.
Shall I go ahead and build the amp as it is and then think of any modifications after listening? Could any of you suggest a good output transformer for this circuit?
Thanks and regards,
rk
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Old 17th June 2002, 07:22 AM   #9
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Quote:
Originally posted by arkay
Could any of you suggest a good output transformer for this circuit?
Dynaco ST70 OPTs

These R-Core OPTs -- about 1/2 way down the page.

dave
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Old 17th June 2002, 07:53 AM   #10
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Hi Dave,
I visited the link for the R-core OPTs and saw the specs of RX-40-5
Push-Pull 5,000 ohms 40W type Audio Output Transformer. The said OPT has only a single 6-Ohm secondary, whereas in the amp schematic I see the cathodes of 807s connected to the 16 Ohm tap and common in the secondary, apart from 4 Ohm tap grounded through a 1 Ohm 5 W resistor. I know that I am sounding silly, but if I use the said R core OPT, how will the connections be made?

Thanks in advance,
rk
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