Well, I finally "finished" 
the 6B4 single ended amp I have been building (as explained in this thread.
It was a fun experience, punctuated by a couple of weird problems- I blew up a 450V capacitor by putting 500V across it, smoked the 50K hum pots I mistakenly put in (doh, should have been 50ohm) and absolutely messed up the woodwork, which will be fixed later.
The verdict! It sounds great. Very smooth and impressive. Vocals sound very natural and airy. Other music sounds great too. It is still early days, I have only had it running for an hour or so on all new components, so I guess it will burn-in a little. However, listening to a couple of cool CDs, you could really hear good stereo effects, it was nice to listen to.
Tubes are Sovtek 6B4-G, tung-sol 12SL7, and a CV729/5V4G rectifier. They are all scavenged NOS apart from the sovteks which i got from ebay. Transformers are some junk ones I had lying around, and are on a separate chassis. Outputs are Hammond 125ESE.
Cool! I have wanted to build a SET for ages
Updates are definitely planned...
Here are some grainy webcam pics:
the 6B4 single ended amp I have been building (as explained in this thread. It was a fun experience, punctuated by a couple of weird problems- I blew up a 450V capacitor by putting 500V across it, smoked the 50K hum pots I mistakenly put in (doh, should have been 50ohm) and absolutely messed up the woodwork, which will be fixed later.
The verdict! It sounds great. Very smooth and impressive. Vocals sound very natural and airy. Other music sounds great too. It is still early days, I have only had it running for an hour or so on all new components, so I guess it will burn-in a little. However, listening to a couple of cool CDs, you could really hear good stereo effects, it was nice to listen to.
Tubes are Sovtek 6B4-G, tung-sol 12SL7, and a CV729/5V4G rectifier. They are all scavenged NOS apart from the sovteks which i got from ebay. Transformers are some junk ones I had lying around, and are on a separate chassis. Outputs are Hammond 125ESE.
Cool! I have wanted to build a SET for ages
Updates are definitely planned...
Here are some grainy webcam pics:
Attachments
Well, the woodwork will be redone, I think I'll go to a picture framer to get the corners done nicely.
And yes that is a choke on the main amp chassis, it had a badge 15H 100 which I assume means 15 Henry 100mA. I am a bit over that (~130mA) but never mind.
This amp seems to get better as I use it... Nex thing might be some decent cathode bypass caps, I am just using random electros at the mo... Might try bypassing with some film caps.
Schweet! I am especially pleased, because this whole thing has cost me only a couple of hundred, really cheap, and sounds great.
Power out is only about est. 2-3W at the moment, as the B+ is a bit low (375V) but I can dial it up a little.
And yes that is a choke on the main amp chassis, it had a badge 15H 100 which I assume means 15 Henry 100mA. I am a bit over that (~130mA) but never mind.
This amp seems to get better as I use it... Nex thing might be some decent cathode bypass caps, I am just using random electros at the mo... Might try bypassing with some film caps.
Schweet! I am especially pleased, because this whole thing has cost me only a couple of hundred, really cheap, and sounds great.
Power out is only about est. 2-3W at the moment, as the B+ is a bit low (375V) but I can dial it up a little.
Hi ShiFtY!
Very nice amp! Since the SovTek 6B4G's use the same plate structure as their 2A3's, I have a pretty good idea what it sounds like - and it's a very nice sound!
You'll find that the sound will be even smoother and even less granular with good cathode bypass caps (or even okay ones - I used Parts Express's .1uF Dayton Film and Foils) and bypass caps on the PS (make sure you allow plenty of surge voltage overhead on this though). You could also try some RF suppression with "X" rated .1uF safety caps (do NOT use anything but name brand "X" rated UL Listed safety caps for line filtering! I use Panasonics from Digikey FWIW). Also, even an inexpensive ferrite choke from Radio Shack on your grounds at the high quality ground point may be useful - it was here, but RFI is a real hassle here).
All the best,
Morse
Very nice amp! Since the SovTek 6B4G's use the same plate structure as their 2A3's, I have a pretty good idea what it sounds like - and it's a very nice sound!
You'll find that the sound will be even smoother and even less granular with good cathode bypass caps (or even okay ones - I used Parts Express's .1uF Dayton Film and Foils) and bypass caps on the PS (make sure you allow plenty of surge voltage overhead on this though). You could also try some RF suppression with "X" rated .1uF safety caps (do NOT use anything but name brand "X" rated UL Listed safety caps for line filtering! I use Panasonics from Digikey FWIW). Also, even an inexpensive ferrite choke from Radio Shack on your grounds at the high quality ground point may be useful - it was here, but RFI is a real hassle here).
All the best,
Morse
Well there are probably a lot of factors which affect the sound but here goes:
I usually use a 6V6 PP amp that I have rebuilt. It has extremely high quality output trannies from the 60's and has a really crisp sound, very high resolution. I think the PP has more finer detail, and more immediacy to the sound. It still has a lot of warmth to it however. It also has gobs more power (About 20W or so in class AB) If I were just building one amp, I would go for PP with a little negative feedback. Also rebuilding a vintage amp is heaps of fun!
The SET amp sounds a little slower, but more "natural" with vocals and soft music. I was surprised actually, the difference isn't as huge as you would think. A world away from SS though. The SET has a massive stereo effect, you can really tell where the sounds are coming from.
I am going to try a few changes, see whether I can fine-tune it a little. BTW it still has enough power to drive my 89dB speakers It is a really nice "sit down and listen to music" amp.Yeah they rock. A little rolled off at the top, but that could be other components in the circuit (plus I like my treble ear-bleeding
) But I cant imagine that you could need much more out of a transformer (maybe more power handling I guess). Thanks for the suggestion. I will definitely try a few things like this once I get it a bit more sorted.
Well, this amp just keeps sounding better. It has smoothed out considerably over the last few days. The voltages are spot on, and so is the sound! It really does sound great.
There is an ever so slight 50Hz hum which appears a second after turn on (way before the B+ comes up) which I am assuming is the 6B4 filaments. I am going to try rectified DC to see whether I can cure it. It's not too noticeable, except in the silence between tracks.
And the treble seems to be well balanced now (I suspect it was the capacitors "running-in")
I think next I will try a little negative feedback just for a laugh. (OH NO!!! Heresy)
There is an ever so slight 50Hz hum which appears a second after turn on (way before the B+ comes up) which I am assuming is the 6B4 filaments. I am going to try rectified DC to see whether I can cure it. It's not too noticeable, except in the silence between tracks.
And the treble seems to be well balanced now (I suspect it was the capacitors "running-in")
I think next I will try a little negative feedback just for a laugh. (OH NO!!! Heresy
)hf roll off...
I've been thinking, with all these transformers out there who are not so great for highs would the following idea work ? - basically add another transformer in parallel specifically for the highs only (ie. acting as a hi-pass filter). This way you can have a bi-wire/bi-amp hybrid. The thing is that a high-pass tx need not be so expensive, a small ferrite core for example (I've heard that even air-core would work above 10kHz).
Any opinions on this ? Has it got any chance at all of succeeding ?
cheers
Stelios
I've been thinking, with all these transformers out there who are not so great for highs would the following idea work ? - basically add another transformer in parallel specifically for the highs only (ie. acting as a hi-pass filter). This way you can have a bi-wire/bi-amp hybrid. The thing is that a high-pass tx need not be so expensive, a small ferrite core for example (I've heard that even air-core would work above 10kHz).
Any opinions on this ? Has it got any chance at all of succeeding ?
cheers
Stelios
Stelios,
If a particular transformer is down 3dB at 20kHz, filtering the low frequencies out before it will not improve that number.
And, typically, the issue with output transformers is their response way out from audio - since this impacts square wave performance. Also, deep bass loss due to lack of primary inductance, etc, is an issue - but I don't think it's all that tough to find even low cost transformers that are flat at 20k. Is it?
joel
If a particular transformer is down 3dB at 20kHz, filtering the low frequencies out before it will not improve that number.
And, typically, the issue with output transformers is their response way out from audio - since this impacts square wave performance. Also, deep bass loss due to lack of primary inductance, etc, is an issue - but I don't think it's all that tough to find even low cost transformers that are flat at 20k. Is it?
joel
Hi Joel
I think it's quite hard to find a good OT that's flat 20-20k -depending on your definition of cheap of course (I'm a student)
Let me re-iterate my point since I'm not sure it has come across very well. Basically think of it as two OT's connected in the following fashion: one (say ferrite core) feeding, say, a tweeter and the other (say iron core) feeding, say, the mid/woofer. Both OTs' primaries are connected in parallel. The combined primaries then form the load.
So, I then imagine this will form some sort of a crossover. The highs will be unable to go to the tweet. via the iron OT so they will go through the ferrite and vice versa the lows won't go to the tweeter but will go to the mid/woofer through the iron OT.
If this is what you understood I meant then I find it hard to understand your explanation because the highs and the lows, in my scheme, ideally do not affect one another but "travel in parallel but separate ways" (ie. that's what I hope they'll do).
From your post I can vaguely suspect that you thought that I proposed a scheme where the two OT's (ie. their primaries) are connected in series - which is not what I had in mind. I also do not propose that the two OT's are cascaded ie. that the secondary of the first OT one feeds the primary of the second OT and then the latter's secondary feeds the speaker.
Just thought of posting this for the sake of clarity.
kind regards
Stelios
I think it's quite hard to find a good OT that's flat 20-20k -depending on your definition of cheap of course
(I'm a student)Let me re-iterate my point since I'm not sure it has come across very well. Basically think of it as two OT's connected in the following fashion: one (say ferrite core) feeding, say, a tweeter and the other (say iron core) feeding, say, the mid/woofer. Both OTs' primaries are connected in parallel. The combined primaries then form the load.
So, I then imagine this will form some sort of a crossover. The highs will be unable to go to the tweet. via the iron OT so they will go through the ferrite and vice versa the lows won't go to the tweeter but will go to the mid/woofer through the iron OT.
If this is what you understood I meant then I find it hard to understand your explanation because the highs and the lows, in my scheme, ideally do not affect one another but "travel in parallel but separate ways" (ie. that's what I hope they'll do).
From your post I can vaguely suspect that you thought that I proposed a scheme where the two OT's (ie. their primaries) are connected in series - which is not what I had in mind. I also do not propose that the two OT's are cascaded ie. that the secondary of the first OT one feeds the primary of the second OT and then the latter's secondary feeds the speaker.
Just thought of posting this for the sake of clarity.
kind regards
Stelios
A-ha - so you mean that the original OT with the weak highs basically shorts at hf ? if so then I see Joel's point now.
Then yes, adding another OT in parallel won't work.
The only thing one can do then is add another choke in series to the first OT and make that a wide bandwidth one.
Then take the second OT connect in parallel to the choke-OT combo. Essentially we move the a crossover from the speaker to the amp.
But that's a different can of worms and potentially more expensive to implement well than getting a good OT in the first place.
Point taken.
Cheers
Stelios
Then yes, adding another OT in parallel won't work.
The only thing one can do then is add another choke in series to the first OT and make that a wide bandwidth one.
Then take the second OT connect in parallel to the choke-OT combo. Essentially we move the a crossover from the speaker to the amp.
But that's a different can of worms and potentially more expensive to implement well than getting a good OT in the first place.
Point taken.
Cheers
Stelios
no money for hi-fi...
Really? Well, how about $35? You can get a Hammond 1609 for push pull, or a 125ESE for single ended in that price range. Yes, you need to limit the current flow through the 125ESE, and keep your Rp low to get it nice and flat - but it is easy enough to do. For only $89 I can also get a 1628SE which is flat to 20kHz, at 25 watts!
As far as the parallel/series/multiple OT's/amp crossover idea you had - it left me thinking why bother? Keep it simple. That's what will set you, and your sound, apart from Sony Corp.
stelios said:Hi Joel
I think it's quite hard to find a good OT that's flat 20-20k -depending on your definition of cheap of course
Really? Well, how about $35? You can get a Hammond 1609 for push pull, or a 125ESE for single ended in that price range. Yes, you need to limit the current flow through the 125ESE, and keep your Rp low to get it nice and flat - but it is easy enough to do. For only $89 I can also get a 1628SE which is flat to 20kHz, at 25 watts!
As far as the parallel/series/multiple OT's/amp crossover idea you had - it left me thinking why bother? Keep it simple. That's what will set you, and your sound, apart from Sony Corp.
Yep, I've often thought of these - hence why I'm reading this thread
My problem is I like the 845 so I'm not sure how successful it would be to mate it with the 125/125SE.
Anyone know how this OT performs in high impedance configuration ?
better or worse than low impedance...?
Anyway, for a while I've been thinking of winding a mains toroid myself to serve as an OT as well but that's not necessarily a cheaper option..
regards
S
My problem is I like the 845
so I'm not sure how successful it would be to mate it with the 125/125SE.Anyone know how this OT performs in high impedance configuration ?
better or worse than low impedance...?
Anyway, for a while I've been thinking of winding a mains toroid myself to serve as an OT as well but that's not necessarily a cheaper option..
regards
S
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