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Old 17th June 2002, 08:03 AM   #11
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Quote:
Originally posted by arkay
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.
You are right. That one would only work if you were converting it to cathode bias.

This is the 807 (1625 is just a different heater V) that i like best... i'd probably use something like 6922 or 6H30 on the front.

Click the image to open in full size.

I have the whole article scanned if you are interested.

dave
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Old 17th June 2002, 08:31 AM   #12
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<b>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?</b>

Yes, just build it for now as it is in the schematic, and then tweak. I've re-run the power supply sims and I keep getting a bit higher B+ for each channel, around 250-260V. This is a lower value than I probably would have picked for the circuit. However, the amp has fixed bias, and the tubes are run well within their ratings, so I wouldn't have any concern. It might be worth setting the bias to 17-18V if you end up with the B+ around 270V. If you find a transformer with a secondary as low as 200V, it should still work fine.

The output transformer is spec'd as 5k - 7k plate-to-plate, so look at

<a href="http://www.lundahl.se">Lundahl</a> LL1663 (5k) or LL1620 (6k) available from <a href="">diyparadiso in Italy</a> for $US70 and $US104 respectively. They also have a good guide to chosing trannies and are nice guys to deal with.

Hammond 1650F - 5k, 30W $US50~
Hammond 1659G - 6k6 35W $US55~
Hammond 1650P - 6k6 60W $US72
Prices are from <a href="http://www.tubesandmore.com/">Antique</a>

<a href="http://www.magnequest.com/">MagneQuest</a> can also do excellent PP trannies. Mike has all the winding specs for Dyna, Peerless and others. His reputation is very good.

Bartolucci have a great reputation, but are quite expensive. Available from <a href="http://www.audiokit.it/ENG/Frames/Introduction1.htm">Audiokit Italy</a> who also have a lot of great parts and are friendly. I'm sending off another order to them tonight.

<a href="http://www.tektron-italia.com">Tektron-Italia</a> also make PP trannies, but you'd have to email Attilio and ask for specifics. Their SE kits and transformers have a great reputation. I've ordered some chokes from him based on his reputation, but they're in transit so I can't give personal opinions yet.

In pre-loved trannies you could also look at any of the Fisher etc iron of about 30W that ran tubes like 7591/7868 or 6V6, 6L6, 5881 etc. These are often 6k6 primaries and ran the tubes at similar levels. For example the ones out of my 800B would probably work fine.
John Atwood did a series of performance tests on mainly older transformers about a decade ago. Read all about it <a href="http://www.one-electron.com/Trans_Tests.html">here</a> to get some ideas of whats suitable and good.

I (personally) disagree with Dave's suggestion of the A470. The primary Z is too low for my taste at 4k3, and I would go for something at the higher end, but the 470, or the Magnequest re-issues are excellent transformers, and would probably work very well in the circuit. If you find some of these at a good price, try them as they can usually be resold easily for little or no loss.

The R cores Dave suggested look very nice too. Dave have you or anyone you know used these R cores? The price and specs are good.

Handwound transformers in the US (qqqqqqqq on ebay) have been getting a fair bit of bad press on AA and other forums of late due to poor specs. They are very cheap, but I'd spend my money elsewhere.


Cheers
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Old 17th June 2002, 08:59 PM   #13
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Quote:
Originally posted by Brett
I (personally) disagree with Dave's suggestion of the A470. The primary Z is too low for my taste at 4k3, and I would go for something at the higher end
I'll keep that in mind. Someone mentioned to me that they might work -- i will be using mine with a set of red-bottle Tesla EL-34s.

Quote:
The R cores Dave suggested look very nice too. Dave have you or anyone you know used these R cores? The price and specs are good.
A couple guys on the JoeList ordered some and early reports are promising, but no in-depth reports yet.

Quote:
Handwound transformers in the US (qqqqqqqq on ebay) have been getting a fair bit of bad press on AA and other forums of late due to poor specs. They are very cheap, but I'd spend my money elsewhere.
A thread on this recently also on the JoeList. A number of people have dealt with him, about 75% seemed to not real pleased.

dave
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Old 18th June 2002, 07:14 AM   #14
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Hi,
I was going through the Hammond catalogue, and found the following PP - OPT.
Model : 1645
Audio Watts : 30
Primary Impedence : 5000 ct
Max. DC per side : 128 mA
Secondary Impedence : 4-8-16 Ohms and 70 V

The link to the schematic of the OPT as shown in the catalogue is given below:
Click the image to open in full size.

The secondary connection for 4 Ohms load is:
Click the image to open in full size.

The secondary connection for 8 Ohms load is :
Click the image to open in full size.

The secondary connection for 16 Ohms load is:
Click the image to open in full size.

Can I use this OPT?
If so, how do I wire the feedback?

Any help would be highly appreciated.
Thanks and regards,
rk
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Old 18th June 2002, 11:25 PM   #15
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Feedback is typically wired from the 16 ohm tap to the cathode of your driver tube. If your schematic does not give you a value for the resister in the circuit putting a 100K pot in and turning till it sounds the best, removing it and measuring the resistance will get you very close.
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Old 19th June 2002, 05:51 AM   #16
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Hi all,
Looks like I phrased my doubt in a manner which is not clear.

I want to use Hammond OPT model 1645. The specs for the same are given in my previous post. Is this OPT OK?

My doubt is, if I wire the secondary of the OPT to connect to 8 Ohms load, (as shown below), is it correct to connect the cathode of the top 807 to the Black coloured wire in the secondary, and the cathode of the bottom 807 to the yellow coloured wire in the secondary?

To which wire in the secondary should I connect the 1 Ohm 5W resistor, the other end of which is grounded?

Any assistance would be highly appreciated.
Thanks in advance,
regards,
rk
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Old 27th June 2002, 01:38 AM   #17
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The Hammond will be at its best only when you use the 16 Ohm tap. When wired for 16, the 4 Ohm tap is the center tap. If you're using 4 or 8, you'll only load half of the secondary (Since they're intended to be in parallel) and coupling will suffer. So the high frequency response will not be as good as it could be. But it may be more than good enough - I don't know.

I've considered building this - I might yet - and I'll probably use Hammond Iron, unless something vintage turns up. A dozen 6BG6s (like an 807, but with octal base) are looking for work...
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Old 14th March 2011, 04:47 PM   #18
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Planet10,
Dave it has been almost 9 years since you posted the 807 schematic in this thread. Are you still able to provide the complete article on that amp.
I would very much like to give it a read and try this. The layout is not too complicated and would work nice for a first time tube build. I need to get something assembled so as to begin learning the particulars of tubes. I do not need to have something overly complex for a first go.

Tad
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Old 14th March 2011, 05:06 PM   #19
ryuji is offline ryuji  United States
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If you are interested in single ended class A, check out the schematics to my amplifier i built. I can give additional information if you need any. I did 5 channels but you can do as few/many as you want with my design as long as your power supply can keep up

5.1 channel 807 SE amplifier
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Old 16th March 2011, 06:52 AM   #20
Jaap is offline Jaap  Netherlands
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Quote:
Originally Posted by planet10 View Post
You are right. That one would only work if you were converting it to cathode bias.

This is the 807 (1625 is just a different heater V) that i like best... i'd probably use something like 6922 or 6H30 on the front.

Click the image to open in full size.

I have the whole article scanned if you are interested.

dave
A more modern version (uses current sources) of this topology is the PP-1c amp of the late Allen Wright. If you change the cascodes of triodes to pentodes you have the Engineers Amp of Pete Millett.

I built both and they are both great.
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