Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: PharmRock on January 23, 2022, 09:06:44 am
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JTM build is going to go through the power-up procedure today. Before I do, I was considering changing my ground point for my presence control, which in effect is the ground for the PI, correct?
Anyways, out of convenience, I've grounded the presence control to the preamp buss. However, the location of the bias feed to the NFB resistor on my board would make it where I could run shielded cable to carry the NFB from the NFB resistor to the presence control, and use the shield to carry the ground from the presence control back to the bias feed turret.
Any ill effects from doing this? Is the power amp ground a more suitable place to ground the presence control, or am I overthinking this and should just leave the pot grounded to the preamp buss?
Thanks
edit: I know the bias feed is grounded to the power-amp ground via the R3 "idiot resistor", that's why I was thinking of using it instead of going "directly" to the power amp ground buss on the board (where the 1-ohm bias resistors and bias caps are grounded).
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Show us a schematic so we can understand what you as saying.
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As I see it, using Merlin’s rationale, the CCW terminal of ghe presence control should refer to the -ve terminal of its HT node decoupling cap.
Also referring there are the mid pot and speaker jack.
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Schematic attached.
V1 and V2 share a filter node, and are currently grounded at "preamp ground", which is a "suspended" buss bar that is bolted to the chassis near the input jacks. This includes the mid control ground.
V3 (the PI) is currently grounded via the presence control to the pre-amp buss as well.
Power tubes are grounded via 1-ohm resistors to a ground buss on the board which is also shared with the bias supply network (the typical Hoffman board layout). A single ground wire runs from this to a star ground on the chassis which includes the reservoir cap ground, screen filter ground, PI cap ground, elevated heater supply ground (see schematic), and HT-CT.
Speaker jack is grounded to pin 8 of V4 (which then runs to board to 1-ohm bias resistor).
Mains is grounded separately via a dedicated bolt.
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Sorry...use this one.
I forgot to reposition the power amp ground point.
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edit: I know the bias feed is grounded to the power-amp ground via the R3 "idiot resistor"
What the hell is an "idiot resistor"? R3 is a 1M connected on the input jack.
Do not connect the presence pot to the bias circuit.
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Sorry about that. Dealing with some distractions this morning.
I was referring to this statement by Merlin when discussing bias supplies: http://www.valvewizard.co.uk/bias.html (http://www.valvewizard.co.uk/bias.html)
"It is good design practice to limit the range of voltage adjustment so it is impossible to reduce the bias to nothing (0V). This at least gives a modicum of protection against accidental or unskilled adjustment. This limit is provided by R2 which forms an endstop for P1, also called an ‘idiot resistor’!"
Which obviously I got wrong by calling it R3, and, well....I screwed up that explanation.
I was thinking that grounding the presence via the bias supply would be ok, in that when implementing the LarMar PPIMV, the bias supply is also used for the ground path for the shield.
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Well,
After a little more reading and comprehending, I realize my original question was not only poorly worded, but just plain stupid.
I ended up grounding the presence pot directly to the PI cap.
Fired her up today. I lost the 50/50 bet on the OT primaries and after going in and swapping those around, all is good.
Voltages look pretty good, although about 10-15% higher than expected across the board due to the ClassicTone PT HV being spec'd at 690VAC vs. 650VAC that is typically called for. Screens are within 1VDC as the plates (~445 VDC).
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Let’s see some pics of the build!
Build yourself a vintage voltage box with a 12 volt filament transformer rated at 3 amps to bring the voltages down. Your essentially dropping 12 volts off your 120 volt wall, and of course, dropping the secondary voltage as well.
http://robrobinette.com/images/Guitar/5E3P_Build/Buckminister_Bucking_Transformer.png
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Let’s see some pics of the build!
I concur!! Let's see some pictures, and or video's and let's hear it...
G.
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Let’s see some pics of the build!
Build yourself a vintage voltage box with a 12 volt filament transformer rated at 3 amps to bring the voltages down. Your essentially dropping 12 volts off your 120 volt wall, and of course, dropping the secondary voltage as well.
http://robrobinette.com/images/Guitar/5E3P_Build/Buckminister_Bucking_Transformer.png
Wouldn't this affect all of the AC in the amp, including the heaters? My heaters are spot-on at 6.3VAC. I'm thinking the PT HV is just over-spec'd a little.
An approach I was thinking of using is a string of Zeners off the HT-CT.
unfortunately the PT rectifier winding is only rated at 2A, otherwise I would stick a 5U4 in there which would probably take care of everything.
But, there's plenty of 50W Marshalls out there running at 450V on the plates, at least from what I've seen posted. I may leave B+ as it is, and see if I can bring the screen voltage down a bit. Some of the schematics I've looked at have anywhere from zero dropping resistor to the screens up to 8.2K (not including the 470R on the socket). I might also beef up the PI dropping resistor. It is currently 10K....I think going to ~13K will get my 12AX7s in the typical range for this amp.
I'm going to look at things a little more and see if I can figure it out.
I'll post pics once I am done tweaking, along with the schematic and layout.
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Wouldn't this affect all of the AC in the amp, including the heaters? My heaters are spot-on at 6.3VAC.
Yes, but, not very much with the low acv heaters. The hi acv will be affected much more. And most tubes are rated at +/- 10% of their heaters ac/dcv. So 6.3 heaters can be run at 5.67 to 6.9 ac/dcv.
....unfortunately the PT rectifier winding is only rated at 2A, otherwise I would stick a 5U4 in there which would probably take care of everything.
EL34's can easily handle 450dcv. Just make sure you use screen grid R's.
You could try a 5V4, they still make them.
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Wouldn't this affect all of the AC in the amp, including the heaters? My heaters are spot-on at 6.3VAC.
Yes, but, not very much with the low acv heaters. The hi acv will be affected much more. And most tubes are rated at +/- 10% of their heaters ac/dcv. So 6.3 heaters can be run at 5.67 to 6.9 ac/dcv.
....unfortunately the PT rectifier winding is only rated at 2A, otherwise I would stick a 5U4 in there which would probably take care of everything.
EL34's can easily handle 450dcv. Just make sure you use screen grid R's.
You could try a 5V4, they still make them.
Thanks for the info Willabe....the 5V4 may be an option.
This amp is running KT66's, but I've seen a few schematics which have the higher B+ PT running these tubes, like the one Mark Huss has on his site (attached...his schematic shows 700VAC HT, mine is 690).
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Unless in class A, the HT voltage at idle is neither here or there, somewhat irrelevant apart from setting the bias appropriately.
Because when generating output power, the valves will draw much more current which causes the HT voltage to sag.
It’s the HT VDC (ie at OT CT and the screen grids) at full output power (max unclipped output power) that matters in regard of the loadline.
There’s no set amount of sag, every amp responds differently. Though Marshall type HT supplies are typically a fair bit saggier than Fender.
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Unless in class A, the HT voltage at idle is neither here or there, somewhat irrelevant apart from setting the bias appropriately.
Because when generating output power, the valves will draw much more current which causes the HT voltage to sag.
It’s the HT VDC (ie at OT CT and the screen grids) at full output power (max unclipped output power) that matters in regard of the loadline.
There’s no set amount of sag, every amp responds differently. Though Marshall type HT supplies are typically a fair bit saggier than Fender.
thanks PDF...
I'll try to post some voltages later today, if you guys could give them a look-over and let me know if you see anything out of place that would be great. I did 3 readings over a 1 hour period after the amp had already warmed up a while. They were pretty much within 1-2% of each other. I'll report the averages.
The amp sounds good, but I only did a few little strums here and there, so there is likely some tweaking to do. Mainly just observed it for a while to make sure it didn't catch on fire. Once I get things the way I like them I will implement the LarMar and possibly an effects loop, but those are both down the road.