 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
174.51.102.111
Now that I have the HK W-5m amplifiers to work on, I'm finding the transformers are OK (16458 OPTs). In other words, the problems aren't there. I'm getting undistorted music but keeping supply at 112VAC. B+ is about 520VDC.
I have parts on the way: SDS lab boards (will clean up everything a lot.) New rectifier socket. new 1uf coupling caps. Russian power tubes.
1) I have read you can remove the .1uf cap at the input and a 15K R there. The need for them is obsolete w/modern preamps and sources. Is that true?
2) Can you use a 5AR4 (such as Sovtek) instead of the 5R4? Is there an advantage?
3) How precise do feedback resistors need to be? these measure off by 20%.
4) Has anyone used a tried and true thermistor after the switch, into the power transf primaries?
If everything goes well, I may try the 6CG7 conversion.
Thanks for any advice.
Peter
Follow Ups:
I was wondering...
After removing the 0.1uf input capacitor, which of the 15K input resisters did you remove? Is 470K attached to the input jack or the grid of V1?
Did you insert a CL80 thermister between Pin 2 of V5 and the choke?
Thanks. Rich
Pster
Everything Steve is spot on.The 6cg7 is very worthwhile..I used to use the sds boards to clean things up as far as appearance but I later found that using a combo of film caps or motorruns in the power supply can be a huge advantage..If you are going to sell the amps,the sds boards are great.
One thing I would like to mention and that is,for the 1uf couplers I always use a pair of .47s for each 1uf cap...The 5ar4 will raise the B+ even more and you don't want to do that.
When problems become increasingly difficult,we can always rely on our collective knowledge and seek relief.
1) I have read you can remove the .1uf cap at the input and a 15K R there. The need for them is obsolete w/modern preamps and sources. Is that true? The cap is there to block DC from the preamp output. If you know there is no DC at your preamp output then the cap is not needed.2) Can you use a 5AR4 (such as Sovtek) instead of the 5R4? Is there an advantage? Don't do this. The W5 PS was designed around the relatively high voltage capabilities and high voltage drop of the 5R4. You might get away with an old stock 5AR4 AND circuit mods to lower the otherwise resulting high output voltage but a Sovtek will not survive in this app.
3) How precise do feedback resistors need to be? these measure off by 20%. Quite precise esp if you're working with stereo pair: these play a large role in setting overall gain of the amp. I matched the ones in my W5s to better than 0.5% on a relative basis. EDIT: The 470Ω resistor at the cathode of the first 12AU7 also needs to be precisely matched to the corresponding resistor in the matching amp. To an approximation, the ratio of the FB resistor to the cathode resistor sets the overall gain. (actually more complicated than that but.....) .
4) Has anyone used a tried and true thermistor after the switch, into the power transf primaries?. I have successfully used a GE CL90 in the primary. However, be prepared for alternate advice regarding the part number and circuit location. Many feel the CL 80 is a better choice and that the thing should go at the output of the rectifier.
If everything goes well, I may try the 6CG7 conversion. A definite improvement IMO.
Edits: 06/11/12
Thank you for your helpful advice. While I've restored a number of amps , these are a different sort of beast.
Pete
Post a Followup:
| FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
||||||||||||||
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2013, Audio Asylum, all rights reserved ]
[ Take5 ]
