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 Post subject: Re: 6CY7 Stereo Amp
PostPosted: 20 Dec 2016, 20:54 
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Are there gains to be had by going with a slightly better output transformer with better inductance or would the frequency response be mostly limited by the circuit itself? I have a pair of the little 5W rated Transcendar output transformers and was thinking of putting this amp together with them.


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 Post subject: Re: 6CY7 Stereo Amp
PostPosted: 29 Dec 2016, 16:11 
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kidmoe wrote:
Are there gains to be had by going with a slightly better output transformer with better inductance or would the frequency response be mostly limited by the circuit itself?
Sorry it's taken so long to get back to you on this. The short answer to your question is yes a wider output transformer frequency response will improve performance. But the circuit needs to be matched to the output transformer. Let me explain.

This circuit (without referencing the output transformer specs) has two places that limit the lower end frequency response. These are currently tuned to work well with the Edcor XSE transformer. This transformer is rated from 70Hz to 18kHz (-1dB). Now you can get response lower than 70Hz with this output. However, as you get much below 40Hz the waveform distortion gets pretty high. Frankly I believe that bad bass is worse than low bass so I endeavor to set my breakpoints to keep from driving the transformer very hard below abut 40Hz or 50Hz. With this circuit using the XSE output transformer, the interstage -3dB cutoff is right at 15Hz. The power stage low frequency transfer point (between bypassed and unbypassed) is at 6.2Hz and is just about -2.9dBv. This response produces an excellent tone with controlled rolloff at the lower frequencies.

If looking to go with a different output transformer with lower f-lo performance, you'll want to decrease the interstage rolloff by some reasonable amount. But not so low that bias excursion recovery time becomes an issue. The Transcendar TT-005-OT are rated down to 20Hz. For these (or similar) I would reduce the interstage rolloff by increasing the coupling cap. A 0.033µf coupling cap would lower the interstage rolloff frequency to 4.5Hz (τr ≈ 35ms). I would not go any lower than this for fear of bias excursion recovery becoming an issue. An 0.022µf (6.8Hz, τr ≈ 23ms) would be much better.

I will issue this one caveat. The small Transcendar transformers meet their 20Hz spec by pushing the primary inductance up to 20H. For comparison the Edcor GXSE10-5K transformers (40Hz to 18kHz) have a primary inductance of 5H. The reason I point this out is because this large primary inductance will likely seriously impact the transient performance of the amplifier. If you really feel you need to get that 20Hz response, by all means try the Transcendar outputs. Just realize that one of the really nice things about this amp is how "quick" it is. It has a very good transient response letting very fast and dynamic scores come through unimpeded. It is unlikely to be as quick and detailed with an 20H inductance in the output transformer primary.

You can read a discussion of this at this link: http://www.cascadetubes.com/2014/02/08/a-consequence-of-the-quest-for-20hz-roll-off/

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 Post subject: Re: 6CY7 Stereo Amp
PostPosted: 29 Apr 2017, 11:40 
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Suncalc wrote:
...

Hello Suncalc! I am new to these tube amplifiers but would still want one! Can you build another one like yours? 6CY7 but single channel? thank you! :up:
p.s. I never heard a tube amplifier! many MOSFET's but never a tube amplifier!

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 Post subject: Re: 6CY7 Stereo Amp
PostPosted: 01 May 2017, 07:22 
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Location: France
Hello Suncalc !

I built this amp, it works great !
I saw on the picture that you add a LED in serie with a resistor on the B+ of the righ Edcor.
Did you used a 5mm LED ~2V 20mA ? What value for the resistor ?
On the picture, it seems to be a 1/4W resistor, is it enough for the power dissipation?
Thank's

Pierre


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 Post subject: Re: 6CY7 Stereo Amp
PostPosted: 01 May 2017, 21:14 
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PitchFork wrote:
I saw on the picture that you add a LED in serie with a resistor on the B+ of the righ Edcor.
Did you used a 5mm LED ~2V 20mA ? What value for the resistor ?
On the picture, it seems to be a 1/4W resistor, is it enough for the power dissipation?
Actually I just used a generic 5mm LED. I didn't bother with any design work instead I just threw a 100kΩ resistor in series. At 220V B+, this puts the LED current at about 2.2mA. This is generally enough to get good light out of a generic LED. 2.2mA through a 100kΩ resistor is about 0.48W. I used a 1/2W resistor. It may be marginal but it's been running for many 100s of hours and the LED still works fine.

Generally for small LED indicators I just use a milliamp or two and choose a resistor that gets me in the ballpark. If I were do this again, I'd probably use a 150KΩ or even 200kΩ. It's generally driven by what is sitting close to me on the workbench.

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 Post subject: Re: 6CY7 Stereo Amp
PostPosted: 02 May 2017, 04:40 
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Joined: 07 Mar 2017, 16:35
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Location: France
Thank you for the reply, I will try that.
I have one last question, on the page 1 on this topic you said :
"I found that the B+ voltages were running a little high. As such I added an appropriate dropping resistor in each B+ line to get the plate dissipations back under the maximum of 5.5 watts each."
In my case, B+ voltage is 236V on each channel, should I add a dropping resistor to get 220V ?

Pierre


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 Post subject: Re: 6CY7 Stereo Amp
PostPosted: 02 May 2017, 15:26 
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PitchFork wrote:
should I add a dropping resistor to get 220V ?
Check your plate dissipation. If it's above 5.5W then drop the B+ a little. If not, then leave it alone. My guess is that you're probably fine.

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 Post subject: Re: 6CY7 Stereo Amp
PostPosted: 27 May 2017, 19:17 
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I just thought I'd mention that there is another output transformer from Edcor for this project that will give a little more lower end response. The GXSE5-5.5K-8 is a drop in replacement but has a rated passband 30Hz lower than the XSE10-5K-8 (40Hz vs 70Hz). Given that the XSE yields a lower -3dB frequency of about 32Hz in this design, I am guessing that the GXSE should give good performance down to at least 20Hz (I have not tested it).

And here's the best part, the difference in cost between the XSE and the GXSE is a whopping 38 cents (as of this writing). I was thinking about a pair of tiny 6CY7 mono-blocks. Maybe I'll give the GXSE5-5.5K-8 a try and see how it compares.

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 Post subject: Re: 6CY7 Stereo Amp
PostPosted: 14 Jun 2017, 08:22 
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Joined: 14 Jun 2017, 07:05
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Hi Suncalc

I was thinking of doing this amp as a first tube project, but I have a few questions (if you have the time...):

*What is the gain of the preamp-stage? - I would want to have a pre-out main-in option to allow use of a couple of T-amps for bi-amping (for comparison and when I need a little more power) - is the gain reasonable for this purpose?
*Also, I would like 2 pre-outs if possible? (not sure how it affects the circuit)
*Are there any alternative OTs? the Edcors are not easily found in Europe - here is the list of the main supplier https://www.don-audio.com/Edcor-Transformers_s2 which has the XSE10-8-8K and XSE15-8-5K. I assume the latter is a drop-in replacement, though more expensive... (Hammonds may probably be sourced through Mouser, if there is a suitable model?)
*And lastly, I have a power transformer lying around with 200-0-200 (70mA) - can this be used instead? if so, how should I change the component values to yield the correct B+?

I will likely find many more questions if the project seems plausible :)

thank you for a great project!

BR
Jakob


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 Post subject: Re: 6CY7 Stereo Amp
PostPosted: 27 Jun 2017, 09:08 
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Joined: 07 Mar 2017, 16:35
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Location: France
Hi Suncalc
Thank you for this project, this tube amp works fine.

I have different source to drive this tube amp (computer, Ipod, phone...) and sometimes I'm having issues with treble and bass, I'm wondering if I can add a passive (or active) tone control before the amp.

On this topic, there is two versions, an active and a passive tone control : http://diyaudioprojects.com/Forum/viewtopic.php?f=10&t=358
I think it may be better to build an active to avoid the attenuation.

What do you recommend ?

Thank's

Pierre


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