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 06 May 2021, 02:08

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 8 posts ] 
Author Message
PostPosted: 10 Mar 2021, 20:48 
Offline

Joined: 23 Jul 2020, 12:20
Posts: 11
Hello all

I have built this 6v6 PP amp: https://diyaudioprojects.com/Schematics/DIY-Push-Pull-PP-6V6-Tube-Amplifier/
first of all i must say i did not use a GZ34 but a solid state rectiver but the rest of the PSU is accurate
i do have exactly 320VDC on B+ when the tubes are warmed up in working temparature.
the problem is that if i put the loudspeaker NEGATIVE to ground as shown in the schematic i has a real bad ratatata and hissing sound. but when i put a 39k resistor between NEGATIVE and ground then the amp works relatively fine. however, absolutely NOT 10watt but more like 3 watt when i crank my preamp up to the state where the amp starts distort.
a friend of mine told me the 6SL7 does not have enough power to drive this thing, a 6SN7 might work. i believe him and i will try this. However why do i have these bad noise when i put LS NEG direct to ground? im sure that is not supposed to be ...
any idea? i did measure every single component again and i did not wrong. i'd aprechiate for anything that points me into the right direction

thank you
randy


Top
 Profile  
 
PostPosted: 11 Mar 2021, 01:37 
Offline

Joined: 24 Nov 2010, 14:39
Posts: 38
Location: California
Hi Randy
I built the same amp some years back. It sounds like you have the feedback positive instead of negative. Easiest way to correct that is to reverse the output transformer secondary wires. That is, the lead that is currently connected to speaker positive and the 1K resistor / 1000 pf capacitor becomes speaker negative and is also connected to signal ground. The other lead that you have currently as speaker negative will become speaker positive and also have the 1k resistor / 1000pf capacitor connected to it. The negative must connect to signal ground as it provides the path to ground for the 6SL7 cathodes
The 6SL7 is the correct tube. It actually is much higher gain than a 6SN7. The amp is not a wall-shaker but it sounds good. I never measured it but could be around 10 watts.
At Bruce Heran's suggestion, I changed the two .1 uf coupling capacitors to .22 uf, and the 47 uf electrolytic cathode bypass capacitor on the output tubes to 220 uf. This really helped the bass response.


Top
 Profile  
 
PostPosted: 11 Mar 2021, 03:46 
Offline

Joined: 23 Jul 2020, 12:20
Posts: 11
Jim_O wrote:
Hi Randy
I built the same amp some years back. It sounds like you have the feedback positive instead of negative. Easiest way to correct that is to reverse the output transformer secondary wires. That is, the lead that is currently connected to speaker positive and the 1K resistor / 1000 pf capacitor becomes speaker negative and is also connected to signal ground. The other lead that you have currently as speaker negative will become speaker positive and also have the 1k resistor / 1000pf capacitor connected to it. The negative must connect to signal ground as it provides the path to ground for the 6SL7 cathodes
The 6SL7 is the correct tube. It actually is much higher gain than a 6SN7. The amp is not a wall-shaker but it sounds good. I never measured it but could be around 10 watts.
At Bruce Heran's suggestion, I changed the two .1 uf coupling capacitors to .22 uf, and the 47 uf electrolytic cathode bypass capacitor on the output tubes to 220 uf. This really helped the bass response.


so, you mean i should try this: (??)

- 16ohm speaker out direct to ground (remove the 1000pF/1K)
- speaker negative becomes speaker + and is also connected to the 1000pF/1k which goes into the cathode and also from there to ground
- the current speaker + (4ohm) is connected to ground

I'm confused. The Question is: why does this happen? Is the Schematic wrong? Because if i compare this with other PP Amps the Feedback is always put to ground with a R after a C/R in paralel.
Thank you


Top
 Profile  
 
PostPosted: 11 Mar 2021, 04:44 
Offline
User avatar

Joined: 03 Jun 2020, 11:06
Posts: 118
Location: France
Hi Randy,

@Jim_O is certainly right : the FB is positive, making your amp work as a relaxed oscillator.

Just reverse the C and the 16R connections :
- C goes to the FB line (1nF + R).
- 16R goes to GND.

If you don't want to do this, you can also reverse the primary plates and screen connections of the output transformer : this will do the same effect, and your secondary speaker and FB connections will be correct as is.

Another alternative : you can also swap the grid connections going to the 6V6, with the same result.

The schematic is not wrong, but the polarities of the output transformer varies from one brand to another, and they are not always mentioned. Hammond does it on some of their models, but not all... Others usually give no precisions about this !

Then it's not uncommon to have to test and reverse the connections somewhere in the amp to get the correct phase for negative FB.

-tbln.

_________________
https://guilhemamplification.jimdofree.com/


Top
 Profile  
 
PostPosted: 11 Mar 2021, 09:41 
Offline

Joined: 23 Jul 2020, 12:20
Posts: 11
Problems solved! Thanks to you guys!
Thank you so much!!
Amp sounds Awesome. I just expected a bit more Power as my KT88 SE has about the same Volume Power.
But in General it sounds good.


Top
 Profile  
 
PostPosted: 11 Mar 2021, 18:56 
Offline

Joined: 24 Nov 2010, 14:39
Posts: 38
Location: California
:Glad it worked out!


Top
 Profile  
 
PostPosted: 11 Mar 2021, 21:14 
Offline

Joined: 21 Oct 2020, 14:38
Posts: 12
peroja wrote:
Problems solved! Thanks to you guys!
Thank you so much!!
Amp sounds Awesome. I just expected a bit more Power as my KT88 SE has about the same Volume Power.
But in General it sounds good.


I built the same amp and had the same issue on fire up.

The 6V6 PP is only a couple watts more powerful than the KT88 SE and it takes a fair amount of watts to make a noticeable difference in output.


Top
 Profile  
 
PostPosted: 20 Mar 2021, 12:59 
Offline

Joined: 23 Jul 2020, 12:20
Posts: 11
Im just Dissapointed! The amp burns the Drivertubes after 2 hours. I f*** 5x 6SL7 so far. there is no protecting resitor on the grids and i have voltages on the drivers all over the place. It's impossible to control it. I'm not a beginner and i tried pretty much everything. I'm so dissapointed in this project.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 8 posts ] 

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: Google [Bot] and 58 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