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-   -   Need advice on old 6bq5 amp........... (http://www.diyaudio.com/forums/tubes-valves/38825-advice-6bq5-amp.html)

G 4th August 2004 11:24 PM

Quote:

Originally posted by fdegrove
Hi,



1F into 470K?
0.220F would already be much more low frequency than the OPT can pass.

Don't get me wrong, it will of course work but the higher the RC constant the slower the cap will be.
This could be something worth fiddling a little with. You may find the amp's transient improve with smaller value coupling caps, moreover I'd doubt you'd notice any difference in the bass response.

Cheers,;)

Ok. How about we split the difference and say .47uF?

Eli Duttman 4th August 2004 11:24 PM

Quote:

Any particular reason why the cathode resistors of the 6BQ5s are shared between channels?
Frank,

I own a pair of DECWARE SE84Bs. My understanding is that the shared bias network is attempt to balance the 2 channels. I'm conservative about these things and order my SV83s as matched quads. :) I also order driver tube sets with 4 matched sections.

fdegrove 4th August 2004 11:36 PM

Hi,

Quote:

My understanding is that the shared bias network is attempt to balance the 2 channels.
Well, it would but...It would also give you a totally fake stereo image. Not unlike the crossfeed technique used with headphone amps to emulate a (fake) stereo image.

Quote:

If you do use separate cathode resistors CadaverDog you should double the value to 300 ohms for each resistor as yo will only have half of the current flowing through the resistor and therefore only half of the desired voltage drop (10.5v - 11v).
The most common value I recall seeing and calculating in those EL84/6BQ5 amps is 270 Ohm per cathode.
Also keep in mind to reduce the 1.000 F bypass cap to something a little more sensible: 470 F is already overkill way beyond common sense and will make the amp sound sluggish and lazy unless it has very, very low ESR.

I recall experimenting with those bypass caps, putting really big values in there and ending up without any after rebiasing the tubes a few volts deeper.
Still, a bypass cap on the output tubes would be the wiser thing to do here, omitting the one on the 5687 should improve the amp provided you don't overload it with the input signal.

Give it a try and see what suits your taste/system best...
It's you listening to it anyway.:)

Cheers,;)

fdegrove 4th August 2004 11:41 PM

Hi,

Quote:

Ok. How about we split the difference and say .47uF?
If you carry some spares, try something starting from 0.100F to 0.680F.

In case any of you have an o-scope and see some ringing on the OPTs, that too can be vastly improved by the simple addition of a damping network across the primary of the OPT.
Note that I've no idea how good/bad the DEC OPTs are...

Cheers,;)

G 4th August 2004 11:47 PM

Quote:

Originally posted by fdegrove
Hi,



Well, it would but...It would also give you a totally fake stereo image. Not unlike the crossfeed technique used with headphone amps to emulate a (fake) stereo image.



The most common value I recall seeing and calculating in those EL84/6BQ5 amps is 270 Ohm per cathode.
Also keep in mind to reduce the 1.000 F bypass cap to something a little more sensible: 470 F is already overkill way beyond common sense and will make the amp sound sluggish and lazy unless it has very, very low ESR.

I recall experimenting with those bypass caps, putting really big values in there and ending up without any after rebiasing the tubes a few volts deeper.
Still, a bypass cap on the output tubes would be the wiser thing to do here, omitting the one on the 5687 should improve the amp provided you don't overload it with the input signal.

Give it a try and see what suits your taste/system best...
It's you listening to it anyway.:)

Cheers,;)

In this case it will be CadaverDog listening to it. I'm sure he will appreciate your input though. You are by far the more experienced of the two of us. I just wanted to give him a starting point. Thanks for the reply.

fdegrove 5th August 2004 12:03 AM

Hi,

Quote:

In this case it will be CadaverDog listening to it. I'm sure he will appreciate your input though. You are by far the more experienced of the two of us. I just wanted to give him a starting point. Thanks for the reply.
You're welcome...I still learn everyday and hey, even monkeys fall out of trees, right?:D

I was under the impression you had one of those DECWare amps in the pipelines as well...

Either way it's nice to see others chip in (Oupla...did I say chip?).

BTW, wasn't it you, Gavin, building something along the same lines but using a 12B4A as the output tube or am I mistaken?

Cheers,;)

G 5th August 2004 01:21 AM

Quote:

Originally posted by fdegrove
Hi,



You're welcome...I still learn everyday and hey, even monkeys fall out of trees, right?:D

I was under the impression you had one of those DECWare amps in the pipelines as well...

Either way it's nice to see others chip in (Oupla...did I say chip?).

BTW, wasn't it you, Gavin, building something along the same lines but using a 12B4A as the output tube or am I mistaken?

Cheers,;)

I tinkered around with a design that is similar. I have more designs than I have time or money. Right now I'm focusing on a preamp. The first amp I built was built according to the schematic I posted with a few differences in values. I like the 5687 very much as a driver tube. I understood what CadaverDog was looking for in a design so I thought I would "chip" in. The design is by no means perfect but it is a good simple design that sounds nice. I agree about the 1000 uf cap. I didn't understand about such things at the time. 470Uf would probably be more than enough. The next amp I build will probably be a SE KT88 with a 6SN7 driver but that is a good ways into the future.

cadaverdog 5th August 2004 05:43 AM

Hey guys, I couldn't help but notice your thread about building a 6sn7/kt88 SE amp. Let me know about Your needs as far as tubes go as I have an extensive collection of NOS 6sn7, 8sn7, and 12sn7 tubes. I just acquired a NOS set of GE 6550A's from industrial surplus; they'd probably sound great in that amp.
To answer questions about my username:
Best Buy and Circuit City don't sell tube gear, and to the mass market, it is generally a "dead" technology (hence the username.)


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