SRPP power using power tranny as OPT?

[Brit01]

Looking at alternatives for OPTs. This one uses a 220V/12V power tranny.

Could this be an option as OPTs are not available here. Obviously I'd need to stuthis and recalculate for the tubes I have.

Loftin-White mit PL84 in Pentodenschaltung von Mario Benndorf

[Fuling]

Small toroid power transformers works fine as parafeed OPTs

[Brit01]

I heard that BUT these seem to be totally overpriced here.
Quoted almost 100 USD!!!!!!!!!

standard transformers are really cheap and common here.

[Brit01]

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.

[aardvarkash10]

Shoog is the go-to for torroidal OPTs in parafeed, and has done a fair bit on this topology

[Arnulf]

Originally Posted by Brit01

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.

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.

[Brit01]

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.

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?

[Brit01]

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.

http://www.radioremembered.org/outimp.htm

[Brit01]

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.

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)

so if 2 sv83's in PP with impedance of 7500 equals 15000 I need to use feedback?

[Arnulf]

Originally Posted by Brit01

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

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).