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SRPP power using power tranny as OPT?

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#4
I see this was Geek's post.

Gregg: Could this be built with SV83's from a 6N6P/6N1P SRPP and using a small 220v/12v power transformer?

No idea how many watts it would provide. Haven't done any calculations.

update: reading more posts the general consensus is that these will sound awful hence we have OPT's. I can only give it a go and see for myself.
 
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#6
Brit01 said:
update: reading more posts the general consensus is that these will sound awful hence we have OPT's. I can only give it a go and see for myself.
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Suggested concoction will probably sound awful, yup.

Power transformers don't have to sound awful, just find a decent (dimensions-wise; 20-30 VA or thereabouts for 5-8W output power, scale up proportionally for larger power) example with split (115+115 V) primary and go for straightforward PP. (Something in the range of 6-9V will land you in the SV83 territory, depending on the B+ voltage and desired regime.

What's with this fascination with SRPP in audio anyway ? SRPP is about the most useless topology for driving output transformer + loudspeaker and not very useful for other roles in audio either. It excells at running huge lengths of telco cable.
 
#7
What's with this fascination with SRPP in audio anyway ? SRPP is about the most useless topology for driving output transformer + loudspeaker and not very useful for other roles in audio either. It excells at running huge lengths of telco cable.
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Yes true. I'm only using SRPP for my headphone amps.

If I go down the path of constructing SV83 monoblocks or alike then of course I'll use a different topology.
Sorry for the confusion.

Simple grounded cathode into a PP arrangement with the SV83's probably, with the power amps as OPTs.

I need to buy more power amps for heater filaments anyway so worth a try as opts. These are readily available here.

This is what DIY is all about. Experimentation, tweaking and fun.😉


What about: Transformer 220V/9v 1 Amp. (doesn't specify how many VA). Can I calculate this?
 
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#8
Would this be a correct example?

220/9 ~25 turns to 1

Now, armed with the turns ratio, we can calculate the impedance ratio and the impedance that will be reflected to the primary with a given load in the secondary. Remember we said earlier that the impedance ratio is the square of the turns ratio. With our 25:1 turns ratio transformer in figure 2, the impedance ratio is the turns ratio squared or, 25 X 25 = 625:1. So if the transformer is working into an 8 ohm load, the impedance that will be reflected to the primary will be the impedance ratio (625) multiplied by the load impedance (8 ohms), equal 5,000 ohms. If the load in the secondary is changed to a 4 ohm load, the reflected impedance in the primary would be 625 X 4 = 2,500 ohms.
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http://www.radioremembered.org/outimp.htm
 
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#9
Decaware notes:

For the RMS tests pictured on this page, we used a matched pair of tubes (only one of which is measured at a time during the RMS testing) that tested at 7500 micromhos. This is done on our Hickok model 539a tube tester with the bias setting at 2 volts. We match all our tubes here in direct micromhos using this classic military tester. 7500 is about ave. for the SV83's that we've tested (around 750). Some are as low as 4840 and some as high as 9750.
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So a 220/6v would give me a 10368:8 ratio no? (36*36*8)
And a 220/9v would give me a 5000:8 ratio no? (25*25*8)

Now how does the voltage on the SV83 affect this impedance, for example in a PP circuit?

I'm looking for 7500:8 more or less.

Push-Pull
Let’s use a pair of 6550’s with a transformer with a 5000 ohm primary. Two 6550’s in push-pull have a combined plate impedance of about 10,000 ohms in pentode mode. The transformer has an impedance ratio of 5000 ÷ 8 = 625. The output impedance is then 10,000 ÷ 625 = 16 ohms. In this case it is absolutely essential to employ negative feedback to get the output impedance down. Amplifiers of this nature typically require about 25 dB of negative feedback to get the output impedance down to 0.8 ohms. (25 dB of feedback reduces output impedance 18 times)
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so if 2 sv83's in PP with impedance of 7500 equals 15000 I need to use feedback?
 
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#10
Brit01 said:
Now how does the voltage on the SV83 affect this impedance, for example in a PP circuit?
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Voltage doesn't affect impedance, it affects to position of the operating (quiescent) point and position of the operating point and dissipation curve in turn affect the shape (slope and point where one tube goes into cut-off) of the loadline which then tells you what kind of load (transformer) you're after. This is of course combined with your preference for operating regime (A versus AB).

You appear to be on a very limited budget so you shouldn't be concerned with getting the Z ratio just right down to the last digit (there is no such thing as "just right" as it is extremely unlikely that your tubes would be exactly alike those that the designer of the amplifier you're trying to copy has used).

My guess for SV83 would be somewhere in the vicinity of 8-10K for PP (anode to anode) without bothering to plot the loadline. I never used those, they appear to be slightly weaker cousions of EL84 (EL84 will work well with 7-10K load).
 
#11
Thanks for the advice. Yes limited budget (and limited resources here in Uruguay).

I have 4 x 6080 tubes with a Rp of 280 ohms and 12watt output. These could be a better option no? But need a massive heater current!
TRanny 220/24v = 671:1 impedance. Could be. PP with 6080.

SV83 might be my first attempt.
 
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#12
Its a shame these things do not scale up so well, I had a thought that a GU-50 based SRPP running off about 800V/60mA would be interesting as a parallel feed SE, as the top tube would save any choke/current source issues and the OPT could be a 'cheap' toroid. The only fly in the ointment is the 70V cathode resistors - wasting loads of volts..

70 * 0.060 = 4.2W
70 / 0.060 = 1167 ohms

so each tube would have 400-70 = 330V to play with on that supply.
A bit of a voltage hog but it would be interesting to see how it sounded as a poor mans SE (HT supply excepted).
 
#13
Brit01 said:
Thanks for the advice. Yes limited budget (and limited resources here in Uruguay).

I have 4 x 6080 tubes with a Rp of 280 ohms and 12watt output. These could be a better option no? But need a massive heater current!
TRanny 220/24v = 671:1 impedance. Could be. PP with 6080.

SV83 might be my first attempt.
Click to expand...

I do not recall Uruguay being part of the UK?!

You can always wire the 6080 heaters in series, giving you a reasonable current! Are these maybe for a 6080 power SRPP?
 
#14
I do not recall Uruguay being part of the UK?!
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Your geography lessons paid off! 😉

I'm an Englishman living in Montevideo!😎

You can always wire the 6080 heaters in series, giving you a reasonable current! Are these maybe for a 6080 power SRPP?
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Yes I think a 12.6 or ~25V 4Amp tranny would be the best option for the heaters depending how many I plan to use.
As they are a double triode, maybe just 2 tubes (1 each channel) so a 12.6 with at least 3.5-4 Amps should do.

Probably not a SRPP. Maybe a simple common cathode input (6N1P/6N2P/6N6P...) followed by a PP with the 6080's.
May need a preamp to push them hard enough. (I have a 5687 Aikido amp that would do the trick).
 
#15
Tom (Tubes4e4) wrote a very good article and implimentation of a low powered SRPP Amp using PCL82 or PCL85 tubes 'ECL82 SRPP Flea Power Amp' over on GREGG's site, which was a scaled down version of a high powered amp by (VAPKSE) using 807, KT88 etc over on (Yvesm) site audiyofan.org/

Regards

John
 
#16
Cookj said:
Tom (Tubes4e4) wrote a very good article and implimentation of a low powered SRPP Amp using PCL82 or PCL85 tubes 'ECL82 SRPP Flea Power Amp' over on GREGG's site, which was a scaled down version of a high powered amp by (VAPKSE) using 807, KT88 etc over on (Yvesm) site audiyofan.org/

Regards

John
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I found this thread here: (link) which has the KT88 version, very interesting. Back in the 'olden' days tubes were probably more expensive than OPTs, but today the reverse is certainly true.

The circuit looks reasonable too, except the OPT primary needs to return to the lower KT88 cathode - not ground, and I'd avoid the bypass caps in the first stage.

Looks like a good project - SRPP driving SRPP driving toroids 🙂
 
#17
Brit01 said:
Your geography lessons paid off! 😉

I'm an Englishman living in Montevideo!😎



Yes I think a 12.6 or ~25V 4Amp tranny would be the best option for the heaters depending how many I plan to use.
As they are a double triode, maybe just 2 tubes (1 each channel) so a 12.6 with at least 3.5-4 Amps should do.

Probably not a SRPP. Maybe a simple common cathode input (6N1P/6N2P/6N6P...) followed by a PP with the 6080's.
May need a preamp to push them hard enough. (I have a 5687 Aikido amp that would do the trick).
Click to expand...

Funnily enough I have a couple of 6080 tubes too, did not realise they were doubles - they are a bit grimy too! I think I'd use GU50 tubes but I already have sockets for the 6080 in the 545b 😉

If I were you I'd consider running at about 300-350V (1:1 mains) and using 6N1p SRPP driving 6080 SRPP driving toroid OPTs. I think that would make an interesting amp indeed.. I just need a spare chassis now to try it myself 😉
 
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