• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Amp advice needed for a poor DIY er...

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Dear Ozgur,

I think 2x10W would be quite good for a first tube amp, yes. I think you need to build something and see if you like tube amps or not. So, if I were you, I would build a 2x5W amp and, if I like the result - I would think about something more - if needed ;-) But... what does it mean for the OT? As far as I know the VR20CSE OT I already mentioned allows customization under different plate resistances. Therefore, with such OTs once bought you could do both the first amp and a more powerful one. The manufacturer says these OTs can be used for GM70/GK71, Soviet DHT with max. plate dissipation of 125W... ;-) Well, he needs to know if there is such a plan in advance so that he would use isolation rated for kV operation.

I am sorry to say that I leave for the Black sea this evening and I will be offline for ~10 days, so I will not be able to further support you in that time :-( I will look forward to see your thread development when I get back, of course.
I am sure your answers will not be left unanswered meanwhile, I believe there are a lot experts in tubes here and I suppose they will help you not less than I would.

Yes, I think you can start with EL509. I have to say I'd better check which is closer to 6P45S (6n45C), EL509, EL519... Soviet one is easier for me to get, so I know her better. Again, tube selection is not the first thing here, IMHO, I am afraid that you might have to sometimes wait weeks for OTs manufacture and delivery, so I would consider OT selection in first place. I would suggest that you also gather more peoples' recommendations about OT and plate resistance.

BTW here is a link to one of my favorite articles,
http://www.nutshellhifi.com/library/FindingCG.html
I guess it might also be interesting for you...

Best regards,
Andrey
 
I built my first tube amp using Mikael's SE KT88 schematic.

http://www.diyaudio.com/forums/showthread.php?s=&threadid=59882&highlight=

It puts out around 10 watts per channel, very minimal component and very easy to build. It can power my Wharfedale Pacific Evo 10 which is an 87db speakers. Now am using a Fostex 127e on an inverted BIB speakers (iBIB, discussed on the fullrange/loudspeaker section of diyaudio.com).

This circuit is great for beginner like me.
 
Theyre simply enough projects for me. However my first problem is availability... And the most unavailable thing is the output trafo.. So should I select a OTL one for the first project?

No this is a poor idea as you are attempting one of the most difficult builds around. The chances of expensive failures are extremely high. In the end it is likely to cost you more.

If cost is a real concern, think about a parafeed SEPP amp using EL36's or PL36's and mains toroidals as output transformers. This is a cheap build which should produce a good sounding amp. Do some searches on EL36's and you will come up with a suitable circuit to start from.

Shoog
 
Thanks Shoog,

However I am still not sure on Single Ended Push Pull concept... How it works? If an amplifier runs in single ended how could be a push pull at the same time? (I've asked before)
I am not a tube professor but, as my knowledge push pull works as class AB and single end works as Class A. If you will amplify the upper part of signal with high side and lower part of it with low side then crossover distortion will appears anyway. Am I mistaken?
Anyway, on OTL issue I must be wrong. Thats ok. I am not experienced and transformerless design looks me more healty,
- First I dont use any transformer with SS circuits... Then the otput tranformer looks unusual to me.
- Second, as I know; output transformers automatically limits the lower cut off frequency.
- Third, its hard to find a really good (and of course suitable with your output tube) output transformer. But you say; I can use mains transformers as output in some designs (its a good news).

Now, I need;
- A real class A amplifier,
- 15-20W @ 8 ohm at least,
- Simple circuit design with less adjustment needs,
- With highly avaliable tubes and parts..
Pls...

I dont have any idea with EL36s (or PLs) but if you say theyre ok, then I can try to make one..
 
However I am still not sure on Single Ended Push Pull concept... How it works? If an amplifier runs in single ended how could be a push pull at the same time?

The Single ended Push-Pull is the circuit that is otherwise called SRPP, the circuit that Philips called Single ended Push-Pull is identical to the common SRPP circuit. The SRPP can be seen as either a single ended amplifier with active load or as push-pull amplifier where the tubes are series connected for DC but connected in parallell for AC. Please find an article by John Broskie here that explains a bit more about the SRPP http://www.tubecad.com/articles_2002/SRPP_Deconstructed/index.html


If you will amplify the upper part of signal with high side and lower part of it with low side then crossover distortion will appears anyway. Am I mistaken?

Short answer yes, an amplifier only have cross over distortion if one of the tubes stop conducting close to the zero crossing or if it is very unlinear close to the zero crossing, the classic case for showing cross over distortion is usually a class B transistor amplifier without any idle current, this will show very strong cross over distortion. A class AB tube amplifier can easily be biased so that it will show no measureable cross over distortion, in a class AB amplifier the upper tube will not stop conducting exactly at zero crossing but instead both tubes will conduct in an area around zero. BTW not only single ended amplifiers can run in class A, a push-pull amplifier in class A will give higher power and much lower distortion then using the same tubes in single ended connection.

I agree with shoog that OTL amplifiers are not for the beginner, I also agree with what you say as advantages with an OTL, (better frequency response, lower distortion) but it is also more of a challenge compared to an ordinary transformer coupled amp as you need to run the tubes very close to the absolute limit possible, in my OTL amp the peak current through each tube exceed 2.5A at full output power, this is very different from an ordinary transformer coupled amp.

Regards Hans
 
Ozgur,

Tubes are high impedance devices. It's very difficult to build a tubed power amp that does not use O/P transformers, especially as a 1st project.

You are suffering from a common misconception. The class of operation relates to the duty cycle of an active device, not the topology. It is perfectly possible to build a push/pull amp that operates in Class "A". The reason only Class "A" is found in SE audio circuits is distortion. SE Class "C" (conducting less than 1/2 the time) RF oscillators are common.

May I immodestly suggest "El Cheapo". It's inexpensive, easy to build, and does not require a preamp. The Triad isolation trafo used in the B+ supply has dual primaries, which make it suitable for use in both "240" and "120" V. zones. ANY tube of the 6V6 family can be used in the O/P positions, without circuit changes. The O/P tubes operate in Class "A" most of the time. By the time Class "B" operation sets in, things are too loud for any crossover/notching distortion to be noticed.
 
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