Dynaco low Power Amp - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 7th August 2012, 01:38 AM   #11
diyAudio Member
 
mr2racer's Avatar
 
Join Date: Jul 2009
Thanks everyone. I grounded the secondary and I have music. Its clean and quiet with the sacrificial Radio Shack speakers. I put a volume pot between the input and the preamp tubes. Full volume its fine. Turned all the way down I get a buzz. I must have put the ground for the pot in the wrong place.
  Reply With Quote
Old 8th August 2012, 01:42 AM   #12
diyAudio Member
 
mr2racer's Avatar
 
Join Date: Jul 2009
Hey Guys,

I was wrong, the buzz is not from the pot ground its from the transformer ground. No matter where I ground it I have the buzz. I'm using Hammond transformers. Could that be the cause? It doesn't seem likely? What would happen if I were to remove the cap that connects the cathodes to ground? I would then also have to remove the ground from the secondary. And then I would have to install a cathode resistor?

Another possibility would be to isolate the ground using a pair of reversed diodes to break up the AC?

Last edited by mr2racer; 8th August 2012 at 01:47 AM.
  Reply With Quote
Old 8th August 2012, 02:15 AM   #13
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Whoa, LOL Take a breath and think about all of this, the design works fine without any of the proposed changes. Your last proposed change is for breaking ground loops.

Fill us in on the power supply details.. If you are using a full wave center tapped rectifier for B+ connect the center tap directly to the very first filter cap in the supply.

Hopefully your supply is sufficiently well filtered, (CLC?) the relatively low available loop gain and likely imperfectly matched output tubes probably means the PSRR is not all that high.

How about some pictures both inside and out.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 8th August 2012, 09:29 AM   #14
stajo is offline stajo  Sweden
diyAudio Member
 
Join Date: Jun 2011
Location: Stockholm
It is a floating paraphrase phase splitter and the cathodes are dc referenced through GNF to OPT secondary as pointed out. David Hafler is the constructor and it sounds very good. I have built it.

To kill hum totally I needed more farad on the second C then Hafler did. I used about 150 uF If I remember correctly. And be sure to ground both power and output transformer cores to chassies and power supply return. And depending on what you connect it to, one and only one of the devices shall have direct connection to earth ground.

Last edited by stajo; 8th August 2012 at 09:37 AM.
  Reply With Quote
Old 8th August 2012, 02:23 PM   #15
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Also, the orientation of the output transformer(s) to the power transformer can allow induction of hum from power xfmr core to OPT core(s). The laminations of the transformers should be at 90-degree angle from each other, and not parallel or in line with each other.


OPT core - ==========

pwr xfmr core -
||
||
||
||
||


<---- front of chassis
  Reply With Quote
Old 8th August 2012, 03:02 PM   #16
stajo is offline stajo  Sweden
diyAudio Member
 
Join Date: Jun 2011
Location: Stockholm
Also, the 0.001 cap on the GNF is OPT dependent, so I suggest you leave it out if you dont know the way to measure it up. Its for killing high frec ringing in the OPT.
  Reply With Quote
Old 8th August 2012, 03:23 PM   #17
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Where is the secondary CT grounded? It should go straight to the negative end of the reservoir cap but it was quite common in the valve era to just ground it near the transformer and allow the charging pulses to find their own way via the chassis. A little bit of hum/buzz was regarded as normal in those days.
  Reply With Quote
Old 9th August 2012, 01:02 AM   #18
diyAudio Member
 
mr2racer's Avatar
 
Join Date: Jul 2009
Thanks for all your suggestions so far. I've attached some pictures that might help. The power supply is a C-C-C-L-C. Both dual 50uf caps and the power supply transformer CT are grounded at the same point. Depending on mains voltage I have between 300 and 311 V DC. With .003-.010 ripple. Without an input signal the amp is dead quiet. With music from halfway up to full volume there is no buzz. Below that I get buzz. I'm using an 820 pico farad silver mica in the feedback circuit. The tubes are new JJ 12AX7's and the outputs are a NOS matched quad of Japanese made Magnavox 6V6's.

A buzz is a ground problem correct? I know 60 cycle hum and this isn't hum.

I would appreciate any ideas very much.
Attached Images
File Type: jpg DSCN0317.JPG (808.4 KB, 187 views)
File Type: jpg DSCN0318.JPG (775.7 KB, 177 views)
  Reply With Quote
Old 9th August 2012, 02:31 AM   #19
20to20 is offline 20to20  United States
diyAudio Member
 
20to20's Avatar
 
Join Date: Jun 2010
Location: W-S, NC
We could use a diagram of your amp or a list of the differences in it as compared to the Dynaco.
  Reply With Quote
Old 9th August 2012, 02:46 AM   #20
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
I see that the feed back resistor and compensation cap are located very far away from the input stage, this is a significant mistake - no guarantee that this will help fix this issue but for best performance this network should be right at the input tube socket.

Your volume pot is far too close to the choke and the power switch as well.

Filament wiring to the 12AX7A has large loops around the socket, take a look at the filament wiring sticky for some guidance on how to improve this.

Connect the high voltage secondary center tap directly to the capacitor lug, from there go to mecca ground.

Do not use the chassis as a ground path for the amplifier circuitry - no signal or power supply currents should flow through it!! Connect the mecca ground to the chassis - this should be the one and only point connected to chassis. (Other than the safety ground)

I can see you are using your chassis as a ground return and that the transformer is grounded to the chassis not the cap.

I often use romex for a ground bus.. Only one end is grounded and the supply ground is connected at this point. The bus is routed so as to provide a relatively short low impedance ground for everything connecting to ground.

Not too crazy about the input and OPT secondary grounds scheme, I would probably bring each back separately to the ground bus and ground them all together at the chassis end of the bus. (Sometimes it is helpful to tie the two rca grounds together and bring a single conductor back - I also recommend a 0.01uF ceramic cap from the ground side of the rca jack right to chassis at the jack for RFI.)
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Can I use MANY MANY low power devices for 1 HIGH power amp? SkinnyBoy Solid State 35 8th July 2013 10:44 PM
Dynaco Low Power with 12AY7's mr2racer Tubes / Valves 1 25th July 2012 07:59 PM
preamp supply LOW power LOW noise LOW cost drdagor Power Supplies 1 13th September 2010 11:39 PM
Van Alstine Mosfet 150 Modified Dynaco ST 150 Power Amp Scott Novak Swap Meet 1 7th August 2009 10:45 PM
Need work done on a dynaco power amp ag2217 Tubes / Valves 1 6th August 2007 11:35 PM


New To Site? Need Help?

All times are GMT. The time now is 08:18 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2