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Boyuu EL34 A9 Tube Amp
Boyuu EL34 A9 Tube Amp
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Old 10th September 2020, 11:07 AM   #1281
Tubby23 is offline Tubby23  United Kingdom
diyAudio Member
Join Date: May 2020
Location: London
Hello FLima,

Interesting reading!
Myself, I'd like to try an Rk of 250R but think I'd be taxing the PT too much. It's already quite toasty after an hour or two ��.
Also, my household voltage varies from 235-240 depending if my amp is on, LOL, and if my outlander phev is charging or not so I decided to leave a safety margin. Also, I ordered these OPTs on AliExpress:

£15.65 6%OFF | 10w Single-ended 6P6P EL34 FU50 FU7 tube amp output audio transformers 3.5k output of 0-4-8 Ohm 1pcs
10w Single ended 6P6P EL34 FU50 FU7 tube amp output audio transformers 3.5k output of 0 4 8 Ohm 1pcs|tube amp|audio ampel34 amp - AliExpress

and, again, safety margin. They seem just the fit for our amp and may interest some of you.

I'm happy with the sound but yearn for a little more bass.
It appears brighter since putting in the 270R KRs.

How did you plot the frequency response, if I may ask?

NB: If your capacitors are up to specs, you can simply swap out the original rectifier tube with a 5ar4/gz34 and your rails will gain an extra 20v.
What Hi-fi?!
I can only hear up to 13K!

Last edited by Tubby23; 10th September 2020 at 11:15 AM.
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Old 10th September 2020, 01:18 PM   #1282
Fdlima is offline Fdlima
diyAudio Member
Join Date: Jan 2019
Hi Tubby23,

Thanks for your comments.

I would be very interested in replacing the OPTs since I think they are a weak point in the amp, but I'd need to be sure they will perform better than the originals. Have you installed them yet? if yes, have you noticed improvement in power output and/or sound quality? Please share the results with us. It will be very interesting data for all of us.

Could the lack of bass be because of the speaker? Mine has good bass even with 250 ohms Rk (Actually I wish mine had a bit more high frequency). Does the bass improves with higher value Rk?

Replacing the rectifier tube interest me, but what do you mean by "capacitors are up to specs"? I assuming it is if they're spec'd for the new higher voltage they'll have to operate.

Regarding how I plotted the frequency response graph, I used our good old friend Excel. There is a function in Excel where you can plot graphics using log scale in the horizontal axis.

Here's the link to my Excel. Just fill out the yellow cells and Excel will do the rest. Lima's A9 Amp Frequency Response.xlsx - Google Drive



Screen Shot 2020-09-10 at 9.19.17 AM.png
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Last edited by Fdlima; 10th September 2020 at 01:21 PM.
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Old 11th September 2020, 02:51 AM   #1283
Tubby23 is offline Tubby23  United Kingdom
diyAudio Member
Join Date: May 2020
Location: London
Fdlima, thanks for the Excel sheet - I'll try that.

Regarding caps being to spec - yes, I meant rated voltages.
My c301 and c302 are rated at 450v with c303 being 400v.
With the original tubes and rectifier I was getting B+ of 377v and B2 of 328v. If you stick in the 5ARG only these would rise to about 397v and 348v respectively with another 15v at pin 8 of the rectifier. So best to upgrade your caps.

I only just ordered the transformers - i expect them to take a month at least to arrive, but I'll keep you posted.
What Hi-fi?!
I can only hear up to 13K!
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Old 12th September 2020, 04:11 AM   #1284
Tubby23 is offline Tubby23  United Kingdom
diyAudio Member
Join Date: May 2020
Location: London
Regarding the lack of bass, it's because our OEM transformers are small. If you look around at single ended, single tubed, tube amps, you'll see the better ones have large OPTs even though they can only potentially output up to 10w as ours.
Single ended OPTs must have an air gap in the laminations to prevent the core from saturating from the bias and one way signal current - it's pulsed DC, not AC.
The problem with having the air gap is the transformer contains less iron for a given size.
And ours are already small to begin with so they saturate from low frequencies.
All bigger OPTs will achieve is lower bass not more power.
What Hi-fi?!
I can only hear up to 13K!
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