DIY Audio Projects Forum
 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

DIY Audio Projects Forum

Welcome to the DIY Audio Projects Message Forum. Use these forums to discuss Hi-Fi audio and to share your DIY Audio Projects. Registration is free and required to post messages and view the file attachments. Registration will only take a minute and registered users do not see any advertisements. After you have completed the online registration process, check your email (including spam/junk folder) for the verification email to activate your account. New members are under moderation - so your posts will not be visible until approved by a moderator. See the Read Me 1st, Forum RULES and Forum FAQ to get started on the forum.

It is currently 20 Feb 2019, 06:11

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 45 posts ]  Go to page Previous  1, 2, 3, 4, 5
Author Message
PostPosted: 03 Feb 2019, 19:32 
Offline
Project Author
User avatar

Joined: 06 Apr 2009, 10:08
Posts: 1538
Location: US Pacific Northwest
First the Miller capacitance question. I calculated the C-M of the big triode at about 92 µµf at this bias point. Using the 14.9kΩ output impedance of the driver and the (470kΩ grid resistor) with my original 10kΩ grid stopper, this puts the high frequency rolloff at about 71kHz. This should not be affecting the high frequency response at 20kHz. Your 3.3kΩ stopper puts the high frequency rolloff at ≈97kHz. So the power stage grid stopper is not the issue. The Miller capacitance of the driver stages is only ≈20µµf bypassed and ≈13µµf unbypassed. With the 5kΩ grid stoppers these roll offs should be well above 400kHz. So this shouldn't be an issue.

The first thing I would recommend is to replace the 12BH7 with a 12AU7. Then see what the amp sounds like.

_________________
Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog


Top
 Profile  
 
PostPosted: 03 Feb 2019, 22:12 
Offline

Joined: 11 Nov 2013, 22:35
Posts: 20
Suncalc,
I replaced the 12BH7 tubes with the 12AU7 tubes and there was no change.

I am running this amp straight from a CD player with 2 Vrms signal. I have the 250K (audio) pot at about 9 o'clock for casual listening and at most 12 o'clock for loud listening. Is the resistance of the volume pot causing a problem? If I change to a 100K pot would that help? Could I add a voltage divider between the CD player and amp?

feedback appreciated.

thanks,


Top
 Profile  
 
PostPosted: 04 Feb 2019, 14:11 
Offline
Project Author
User avatar

Joined: 06 Apr 2009, 10:08
Posts: 1538
Location: US Pacific Northwest
The 250kΩ volume pot peaks it presented impedance to the amplifier at 62.5kΩ+Rsource/4. The CD player probably has a very low output impedance (≈32Ω?). So the high frequency rolloff due to the volume pot would still be ≈127kHz. Switching to a 100kΩ pot shouldn't matter.

I'm kind of at a loss for where to go from here. Can you measure the passband of the amp from like 10Hz to 30kHz and see what it looks like?

_________________
Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog


Top
 Profile  
 
PostPosted: 04 Feb 2019, 17:31 
Offline

Joined: 11 Nov 2013, 22:35
Posts: 20
Suncalc,
When time permits, I will do a thorough check of the circuit. Every thing else I have checked so far (voltages, current, and gain) has been right in line. This problem appears to be in both channels and it has had the hf attenuation from the initial turn on but I had temporary speakers and I thought it might improve as it broke in.

Hopefully I will find the fault or at least have more information to work with.

Thanks for your help.


Top
 Profile  
 
PostPosted: 10 Feb 2019, 20:17 
Offline

Joined: 11 Nov 2013, 22:35
Posts: 20
Ever try to do too many things at once?
I think between my wife's comments that it sounded "muffled" and me looking at the frequency response chart in the build, I jumped to conclusions that I may have Miller effect.

I took some time to play some pink noise and measure with a frequency response app on my phone. Not the best, but it does give me an indication. The readings indicate no noticable high frequency roll off. So I took so more time to listen to some different music and see if that told me anything. What I am hearing and probably what my wife is picking up on is a reduced volume at a frequency where some of the vocals fall. As a result on many recordings the vocals sound pushed back in the mix instead of up front. As a result they could be called "muffled".

Current setup is CD player (with 600 ohm output impedance) into the Beast and then to the speakers. Prior to the Beast I was using a preamp with tone controls, but I had to borrow tubes so I took out the preamp. I will get the pre/tone back in line and see if that is the difference I am hearing.

Otherwise, I matched the schematic and values except I used 0.1uF coupling caps instead of the 0.068uF caps. Not sure if my problem could be amp layout, component choices, or the sound of the output tube. I only have one so I do not have another to try out.

As for the lack of dynamics, my other amps have CCS loaded anodes in the driver stage. Would that be an option on the Beast? or choke loaded anodes?

But this amp does some things well. When I listen to Zeppelin, Page's guitar cuts through and Bonzo's drums have great weight. As I have said this amp has grunt.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 45 posts ]  Go to page Previous  1, 2, 3, 4, 5

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: Google [Bot] and 29 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
DIY Tube Projects :: DIY Tube Amp Kits :: DIY Speaker Projects :: DIY Solid State Projects :: DIY IC / Op-amp Projects :: DIY Phono Projects :: DIY Cable Projects :: Hi-Fi Audio Schematics
© diyAudioProjects.com - Terms of Service - Privacy Policy