Small 50C5 Amp improvements - 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 31st July 2008, 05:23 AM   #1
diyAudio Member
 
Join Date: Apr 2007
Default Small 50C5 Amp improvements

Hey folks. This past weekend I managed to pic up a interesting little 50C5 class a amp out of a capehart stereo. I believe it was a phono only unit since its model number is PH 401. I was only $1 and someone had already installed new power supply caps. It sounds decent playing thru some small jbl speakers and into headphones but I would like to improve things.

Instead of a RIAA specific circuit it seams they just put a huge amount of adjustment in the treble and bass adjustments to let you adjust the curve to your own liking. The bass adjustment is in the cathod feedback on a separate winding on the transformer. Looking for opionons of where to start on this. I'm already going to ground the Phono jack input and install the coupling caps on the feed to the 12ax7 grid where it should be. I"d like to remove the equalization and put standard bass and treble adjustments on the input not in the feedback loop.

If I can get it sounding good i'm going to use it for a headphone amp for my PC and just as a small utility amp to lug around.

Recommendations? Thanks

Click the image to open in full size.

Click the image to open in full size.
  Reply With Quote
Old 31st July 2008, 05:41 AM   #2
wa2ise is offline wa2ise  United States
diyAudio Member
 
wa2ise's Avatar
 
Join Date: Apr 2005
Location: NJ
I'll mention the standard warning about such amps, being "hot chassis". You'll need an isolation transformer to make it safe, and this will also help avoid ground loop hum. Once you install this transformer, you could then use a bridge rectifier to feed the amp's heaters and then thru a second rectifier diode, the B+ filter caps. This will allow better B+ filtering, as this filter will get 120Hz instead of 60Hz positive peaks to work with. Less hum. Tube heaters tend to leak less into cathodes when they are at a more positive voltage, so that's partly why I'd have you feed them after the bridge. And it also lets you keep the 12AX7 or similar double triode tube heater near ground voltage. And then the 2nd diode keeps the filtered B+ (which will be approx 120V * 1.4 = 160VDC from overvolting the heaters. Heaters won't mind the unfiltered DC,

Alternatively, you could rewire the heater string so the sequence is 50C5-12AX7-50C5 with the ends wired to the AC output of the isolation transformer. And use a bridge rectifier to get the B+ to feed the filter caps. This would make the 12AX7 heater look to be at about 80VDC above ground, which makes for low heater cathode leakage. This midpoint in the heater string would look like a fake center-tap on the isolation transformer secondary. If you wanted to, you could steal a few milliamps of current off this 80VDC source (filter it though). Hard to imagine why you'd need it though...
Attached Images
File Type: jpg bridgewithheaters.jpg (14.4 KB, 293 views)
__________________
  Reply With Quote
Old 31st July 2008, 04:51 PM   #3
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Yes, definitely use an isolation transformer with this amplifier, as it currently exists it is a safety hazard.

There is no RIAA eq in this amplifier, I'm sure either a crystal or ceramic cartridge was used and the output of such cartridges when run into the proper (high) load resistance and (low) capacitance generally have a relatively flat frequency response when playing records. (The internal cartridge mechanical/electrical transfer function nicely matches the inverse riaa characteristic required for playback.)

Note that this thing currently may also put a small dc current on the input and you should not plug this into anything again until you get the appropriate isolation transformer.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 31st July 2008, 09:36 PM   #4
diyAudio Member
 
Join Date: Apr 2007
Thanks for the input!

Not seen in the picture is the bencch Isolation transformer this unit was plugged into. I've been debating the need to permanently mount a isolation transformer on the chassis as it isn't really a "hot chassis". The chassis is isolated from both sides of the power line. That coupled with the addition of a the wired plug with the ground wire secured to the chassis I was estimating would be enough protection. HOwever, if the general consensous is a iso it required who am I to toy with my life for the sake of a few $$$. I'll get one on order.

wa2wise
Didn't post the PS schematic so you must have these things memorized! Excelent suggestions about reconfiguring heater strings and installing a bridge. Question though. With the 12AX7 in the middle of the string... wouldn't this actual place the 12ax7 heater at or near ground if install across the isolation transfomer and it installed after the bridge it would be at about 80VDC?

kevinkr
I didn't know that. I thought all phono amps/preamps would have to have a RIAA curve. Make my life easy and explains why it seems to perform well with a CD player feeding the circuit as is.


Another thing i've been thinking about doing is intalling Edcor output trannies. However, these trannies do not have the separate output for feedback. Does having the feedback generated from a separate winding have an inherant advantage in this circuit? Is it better just to leave the existing trannies and feedback circuit in place as is?

Steve
  Reply With Quote
Old 1st August 2008, 12:12 AM   #5
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
Steve,

The other posters have covered a lot of ground. One way or another, the active circuitry has to be trafo isolated from the AC mains.

Initially, you mentioned the cap. between the "cold" side of the RCA jack and the chassis. That cap. is essential in a transformerless unit to protect users of the TT. Remember, as shipped by Capehart, that chassis could become deadly "hot". The use of a NFB winding on the O/P trafo is necessary to keep the speaker away from chassis. Notice that the speaker winding "floats".

If we could be certain that the NFB winding could stand some current, placing it in the cathode circuitry would be a good thing to do. My money is on that winding not being able to carry substantial current?

As long as a DPDT switch is used to make/break both sides of the connection to the AC mains, a series heater string is safe enough. Just be sure to install a modern 3 wire power cord.

Speaking of a series heater string, you would do well (IMO) to have only the 50C5 heaters powered that way. Connect pin 3 of the 1st 50C5 to a side of the switched AC mains. Connect pin 4 of the 1st 50C5 to pin 3 of the 2nd 50C5 via 6 μF. of 400 WVDC MPP capacitance. Connect pin 4 of the 2nd 50C5 to the 2nd side of the switched AC mains. The cap. "eats" the extra 20 V., without generating waste heat.

The 50C5 data sheet indicates that approx. 125 mA. of B+ will be needed by the stereoblock. The $29, 100 VA AnTek model AN-1T150 toroidal power trafo will loaf on this job. FWCT rectify the B+ using 2X inexpensive UF4007 diodes. There's more than enough current available in the trafo's heater windings to energize a DC supply for the voltage gain twin triode, should you choose to do so.

You asked for hints on improving sonic performance. Get rid of the nasty tone control circuitry. Add a 7 pin mini socket between the 50C5s for a 0B2 and regulate g2 B+. With a 2 VRMS I/P from a CDP as the driving signal and tone control losses eliminated, you don't need the gain a 12AX7 provides. Switching to a 12AVZ or 5965 as the voltage gain device makes the gain more reasonable and the high gm is insurance against slew limiting associated with a large HF NFB error correction signal.
__________________
Eli D.
  Reply With Quote
Old 3rd August 2008, 12:23 AM   #6
wa2ise is offline wa2ise  United States
diyAudio Member
 
wa2ise's Avatar
 
Join Date: Apr 2005
Location: NJ
Quote:
Originally posted by Eli Duttman
Steve,



As long as a DPDT switch is used to make/break both sides of the connection to the AC mains, a series heater string is safe enough. Just be sure to install a modern 3 wire power cord.


I would avoid running the heaters off the line directly, as the heater-cathode insulation is only good for about 200V. Because of transient spikes from motors in air conditioners, clothes washers, vacuum cleaners and such, the electronics industry found that you need insulation that can take 1.4KVDC "hi-pot".

Quote:
Originally posted by targeteye Question though. With the 12AX7 in the middle of the string... wouldn't this actual place the 12ax7 heater at or near ground if install across the isolation transfomer and it installed after the bridge it would be at about 80VDC?
Heater cathode voltage can be as high as 200V (heater more positive) and it turns out doing that reduces hum leakage from the heater into the cathode. And with the 12AX7 in the middle of the string, string before the bridge, it sees a reasonably constant DC waveform. 80VDC is fine.
__________________
  Reply With Quote
Old 3rd August 2008, 03:33 AM   #7
diyAudio Member
 
Join Date: Apr 2007
thanks again.. I'm going to do the PS mods tomorrow and put in new coupling caps. I'll report back with the results.

Steve
  Reply With Quote
Old 4th August 2008, 04:46 AM   #8
diyAudio Member
 
Join Date: Apr 2007
Today I rebuilt the power supply with a full wave bridge and added about 100uf of capacitance right off of the diodes. I also recapped the amp with Metalized Poly caps. I restrung the heaters like wa2wise had suggested as well.

This all cut down on hum and noise considerably and the amp just sounds better (don't ask my how it sounds better as it may just be in my head).

Eli was spot on with the 12ax7, I didn't have a 12av7 but I did try 12at7 and 12au7a. The liked the 12au7a best and it got quieter still.

Next i'm going to ad shielded cable to run from the tone/volume controls to the 12au7a and ad a shielded tube socket.

After I get that completed I'll report back and post the latest schematic.

Thanks Again for your help.

Steve
  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
isolation transformer for 50C5 amp PRNDL Tubes / Valves 18 17th June 2013 08:26 PM
50C5 conversion PRNDL Tubes / Valves 17 10th May 2007 07:54 PM
quick 50c5 to play around with ThSpeakerDude88 Tubes / Valves 76 26th December 2005 05:29 PM
50C5 add-on amp tubbytwo Tubes / Valves 7 25th November 2004 08:01 AM


New To Site? Need Help?

All times are GMT. The time now is 06:57 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