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Anyone here replaced their tone control PECs in a Voice Of Music 1448 or bypassed the tone controls completely? I tried following the schematic to figure out the circuit and it just makes my head swim.
Pictures with an accompanying simplified yet precise explanation would be a great help. Even direction to where I could find the info would be great. I've already clipped the Tone-O-Matic PEC out of the circuit, but I still hear that there's more room for improvement when I work the tone controls.
Any help would be much appreciated. Thanks.
Larry D.
Follow Ups:
I suspect most PECs use ceramic capacitors. Not so great with high-end audio work. Rebuilding PECs with better components does improve the sound. Customers that perform the same agree- better.I also tried bypassing a well deigned tone section with less than best results for a customer. Sounds better with the tone section. Granted the tone section requires a good tube such as a RCA 12AX7 tall black plate.
Also, I modified customers amps per their request with by-passing the tone section and entire preamp with direct to the phase splitters signal input. Again, not as good per customers review with the rebuilt preamp and tone section. High-end speakers & sources were used in the tests.
So, the customers voice on this matter says no bypassing, rebuild the PECs. But exceptions exist. Dynaco PAS 3X is one.
Edits: 12/09/14
I'd like to use decent quality polystyrene caps or silver micas with copper leads and Takman carbon films on the tone controls. I did this on a Pilot 245 integrated and the results were luscious. I have Hovland .022uf 600V Musicaps in the coupling position in the 1448 and they sound great. The old ceramic discs were testing at .016 to .02 and were leaky according to my Sencore LC-101. It's sounding very good now, but I know it can be better.
Larry D.
About 20 years ago some designers created a fad that no tone controls guarantees better sound. I think the designers found it was cheaper to eliminate tone controls and now are afraid producing a product with tone controls will doom the product due to this ill advised fad.
I think it ridiculous to bypass tone controls. Tone controls allow the system to be tuned for better sound. I have proven this time and time again with quality parts.
I don't think of it as a fad to bypass the tone controls. When you hear a problem with the tone control circuit and do not have a way to fix it, then plan B would logically be to bypass that circuit. I would prefer to stick to plan A which would be to get rid of the PECs and put in discreet components, but I believe plan B would be better than leaving it the way it is.
I don't think it's ridiculous to bypass the tone controls. The old PEC components are going to continue to deteriorate with age and continue to sound worse and worse, so either plan A or B would be better than doing nothing. I've had many a tone control-less preamps that sounded wonderful, so I know that I don't 'need' them, personally.
For now, having the controls at 12 o'clock sounds okay at low volumes, but it seems that they are on their way out when you start to put some power through them. I have two other 1448s other than the one I'm working on and they are not near as bad sounding as this one when you turn the tone controls up. All three have been recapped and the same tubes have been used in each amp.
To sum it all up, I think PECs are just a bean counter's compromise and sound like butt and I'm really looking for answers, not arguments.
Larry D.
Errrr, rebuild the PECs, keep the tone controls. I never build anything to plan B either.
I know it's probably sacrilegious to say so around here, but I actually like the tone controls in the VMs. The key to their use, however, is to forget all about how your tone controls are positioned on any other amp you have.
With other amps, it seems most people start with the tone controls at the 12:00 (noon) postition, which they assume is flat, and they will usually go up from there. With the VMs, I always start with both controls at the minimum and slowly turn them up until I like the sound.
I currently have a 1428 set up here in my home office driving a pair of Dynaco A-25s. The tone controls are set with Bass at 11:00 and Treble at 9:30. I should note that this is nearfield listening at pretty low volume levels. At higher levels I might turn the controls down a bit more, especially the bass.
. . . Charlie
Fla Charlie,It may be that the VOM tone controls work and sound the way they do for you because of the PECs that the signal goes through. I've read that others who have replaced them with discreet capacitors and resistors of decent quality have had great sounding success.
The 1448 is a great sounding little amp, but I hear way more than normal distortion when I turn either the treble or bass past 12 o'clock. Something is not right if that is happening and my suspicion is the PECs because I was easily hearing distortion with both controls at high noon when the Tone-O-Matic PEC was in the circuit, but as soon as I clipped it out the distortion was gone. I think that putting in discreet components in place of the remaining PECs would make this little gem super sweet.Larry D.
Edits: 12/08/14
Hi Larry - If you turn the stock tone controls up too much - meaning, in the same position that works for most other amps - they can (OK, do) sound boomy (bass) and harsh (treble). Components certainly do drift in value, though, so I don't doubt that might have been what caused your problem. It also might explain why my technique works well - maybe the PECs in my amps just haven't drifted as much as yours.
I strongly agree that the best solution would be to duplicate the PECs with discrete components - not to simply remove them or bypass the controls.
I have other projects going at the moment so I can't open one of my VMs up but, looking at a schematic, the PECs don't look all that complicated. Each channel has one that's connected to the Loudness pot and one that's between the Bass and Treble pots. For each channel, the Loudness PEC and the Tone PEC have 2 resistors and 4 caps each. So, that's a total of 8 resistors and 16 caps. I imagine you could build these on perfboard with 1/4 or maybe even 1/8 watt resistors and mica caps. As long as the Loudness pot is OK it seems pretty basic. If the Loudness pot is bad, you might have a problem since it seems to have two loudness taps in each section of the pot (dual sections for stereo), not one. Perhaps that's not as uncommon as I think though??
. . . Charlie
If I find no help with replacing the PECs, then I will try again to figure out the schematic and put in caps & resistors. I'll have to set a good bit of time aside and see how other volume controls are wired up. I'm sure I'll be pleased with the results.
Larry D.
What kind of "help" are you looking for? Your original post indicated that you found the schematic confusing so, perhaps you are just not used to working from schematics. Your other comments seem to indicate that you are experienced at soldering and replacing parts though.
I'd be happy to try to answer any questions you have about the schematic.
Or are you trying to find someone to build these for you?
. . . Charlie
It would be nice to have some pics of where someone else has replaced the PECs with caps and resistors to see how they have them hooked up. It may be that nobody has done this to a 1448 before.
I may just cut out the PECs and try and figure it out from there because the way they're wired up now just confuses me when I try and picture in my mind how the bass & treble should be wired up with discreet parts according to the schematic. I will take pics before I remove the PECs.....just in case I need to re-install them. If I study the amp and schematic long enough I may figure it out anyway, despite the PECs.
I can usually follow a schematic with no problem because I follow the circuit and see (according to the schematic) that a cap is supposed to be next in line and 'voila', its there in the amp, but with a PEC you see nothing that resembles a cap or resistor, and there are connections within the PEC that you can't see, so that throws me off. This may be why they don't get replaced all that often.
Larry D.
Larry - I've thought about replacing PECs too but haven't ever gotten around to it. Today I took a look at the schematic and drew my own, which may be easier to follow. I'll send you an email.
. . . Charlie
I really appreciate any help I can get with this. Since it is not my amp I absolutely do not want to screw it up, but I know (hear) that it needs more work beyond replacing all the electrolytics and coupling caps. Thanks for taking the time to draw it out for me.
Larry D.
Larry - I sent you two messages through AA but I haven't heard back from you. I can't attach the schematic with AA mail, so reply if you still want it.
. . . Charlie
You could send it directly to me since the asylum email is not working. Thanks.Larry
Edits: 12/14/14
I haven't received anything from you yet. And I'd sent you a message also. Maybe you should just copy my email address and send it through your regular email.
Larry
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