Schematic below
High pitched squeal when I Installed the KT66 and turn on the standby switch. Clean signal at the inverter tubes.
Any ideas?
High pitched squeal when I Installed the KT66 and turn on the standby switch. Clean signal at the inverter tubes.
Any ideas?
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Hi, there is a thread (sticky) dealing with this on the JTM45 subforum over at the Metroamp forum, though for some reason I haven't been able to get a hold of the forum of late, not sure what's going on there. Anyway, it seems to be a common problem with new production KT66's in these amps. People stick NOS GEC KT66's in & they,re fine, new production & they get the "standby switch squeal", which is the thread title at Metroamp if you can access it. IIRC it a simple matter of putting 5k6 grid stoppers on the KT66 sockets & neat lead dress. Hope this helps. Cheers
Hi Ivan,
Thanks for the help.
I found the issue. The out put transformer primary wires were reversed, my mistake.
When I got this amp from the owner you would not believe the condition it was in. There was nothing that could be screwed up that was not screwed up!!! This was the worse mess I have ever in my life had to fix.
Also all the original mistakes that Marshall made in the design were copied. I think I have 99% of the redesign work finished. Now it is just a matter of making minor changes to get it tuned up. At least the amp is dead silent with no ground loops.
Actually I do not understand how the amp could work well without the grid stoppers and also work well with the grid stoppers. I have not put the grid stopper resistors in but may install them to see what difference it makes. ATM there is zero no signal noise. There is some noise in V1 and V2 which is hard to hear when I played the guitar through the amp.
I will be glad to get this thing off my bench...lol
Cheers,
Billy
Thanks for the help.
I found the issue. The out put transformer primary wires were reversed, my mistake.
When I got this amp from the owner you would not believe the condition it was in. There was nothing that could be screwed up that was not screwed up!!! This was the worse mess I have ever in my life had to fix.
Also all the original mistakes that Marshall made in the design were copied. I think I have 99% of the redesign work finished. Now it is just a matter of making minor changes to get it tuned up. At least the amp is dead silent with no ground loops.
Actually I do not understand how the amp could work well without the grid stoppers and also work well with the grid stoppers. I have not put the grid stopper resistors in but may install them to see what difference it makes. ATM there is zero no signal noise. There is some noise in V1 and V2 which is hard to hear when I played the guitar through the amp.
I will be glad to get this thing off my bench...lol
Cheers,
Billy
Glad you got it sorted easily. I while back I build an early 50 watt clone on a metroamp JTM45 chassis & ran it for a while without grid stoppers with no problem, however, when I did install them (5k6), measured plate current dropped by about 4mA per tube, showing that there had been some high frequency (in audible) oscillation happening without them. Didn't really notice a difference in tone. Cheers
I should add though, that at the time the amp was in the "break in" phase & as such hadn't really been pushed hard. Cheers
Hi Ivan,
The amp build instructions I am working with indicates the plate voltage should be around 400VDC based on the transformer they used. We have a Merren transformer based on a Drake transformer from back in the day.
The Merren transformer showed a unloaded voltage of 367 VDC. I have no way of knowing what the output of the transformer used to create the build instructions was.
None of this is a big deal except that the trim pot in this amp will not reduce the plate current to less than 46 mA which is a bit to much based on the plate voltage measuring around 380 VDC.
If the 5K6 grid resistors will actually reduce the plate current by 3 or 4 mA in this amp and also remove any unwanted oscillations that would be a good thing.
I think I will install the 5K6 and see what happens. I also will change the value of the trim pot. On another note, I only measured the plate current on one tube so I don't know how well they are matched yet
The owner came by today and was happy with the way the amp sounded so at this point I just want to make sure the amp is stable and safe to use.
I don't find this amp too complex but I also don't think it is a good "first time" amp build for someone with no electronics background or amp building experience.
On the build instructions and the schematic the mains fuse is in the wrong place being on the neutral side of the mains which is not only wrong but dangerous.
Well...such is life. High voltage electronics is inherently dangerous to begin with. Actually I am glad this young man found me before he found the live end of a wire...lol
Cheers,
Billy
The amp build instructions I am working with indicates the plate voltage should be around 400VDC based on the transformer they used. We have a Merren transformer based on a Drake transformer from back in the day.
The Merren transformer showed a unloaded voltage of 367 VDC. I have no way of knowing what the output of the transformer used to create the build instructions was.
None of this is a big deal except that the trim pot in this amp will not reduce the plate current to less than 46 mA which is a bit to much based on the plate voltage measuring around 380 VDC.
If the 5K6 grid resistors will actually reduce the plate current by 3 or 4 mA in this amp and also remove any unwanted oscillations that would be a good thing.
I think I will install the 5K6 and see what happens. I also will change the value of the trim pot. On another note, I only measured the plate current on one tube so I don't know how well they are matched yet
The owner came by today and was happy with the way the amp sounded so at this point I just want to make sure the amp is stable and safe to use.
I don't find this amp too complex but I also don't think it is a good "first time" amp build for someone with no electronics background or amp building experience.
On the build instructions and the schematic the mains fuse is in the wrong place being on the neutral side of the mains which is not only wrong but dangerous.
Well...such is life. High voltage electronics is inherently dangerous to begin with. Actually I am glad this young man found me before he found the live end of a wire...lol
Cheers,
Billy
Hi,
On early Marshalls, they used 1K5 resistors like Fender did; you may want to try it as a value for the KT66 tubes' grids. Also, Marshall at this period was also changing Filter cap values a lot. For some reason, Marshall liked using more filtering for the phase inverter and less filtering for the screens of the output tubes. So, you may want to try 32uf for the 1st filter cap, 16uf after the choke for the screen supply, 32uf for the P.I. and 16uf for the 2 preamp tubes.
You will get "ghosting" of notes with this amp when playing it because of the low filter cap values and because of the 1st 2 preamp tubes B+ supply not being decoupled from each other. But that is the "classic" Marshall sound for these amps.
On early Marshalls, they used 1K5 resistors like Fender did; you may want to try it as a value for the KT66 tubes' grids. Also, Marshall at this period was also changing Filter cap values a lot. For some reason, Marshall liked using more filtering for the phase inverter and less filtering for the screens of the output tubes. So, you may want to try 32uf for the 1st filter cap, 16uf after the choke for the screen supply, 32uf for the P.I. and 16uf for the 2 preamp tubes.
You will get "ghosting" of notes with this amp when playing it because of the low filter cap values and because of the 1st 2 preamp tubes B+ supply not being decoupled from each other. But that is the "classic" Marshall sound for these amps.
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Hi, danfrank is right in the value of the gridstopper used in the early Marshalls, also about the ghosting. If you cant get the required negative voltage for proper biasing as the amp is at the moment, rather than increasing the pot value it is common to reduce the value of the 180k resistor in the AC section of the bias circuit until you get the correct range. I believe that the Metro spec PT (Heyboer) will give a B+ (loaded) of 400VDC - 410VDC, though the originals were in the vicinity of 450VDC. Marstran JTM45 PT's (1202-55) give the correct B+ voltage. I was under the impression that the Merren PT's have high & low voltage secondaries, though I may be wrong about this. Cheers
Hi guys,
Yes, the Merren transformer has a second set of HT taps. I measured the voltage at 436 VAC unloaded. So for sure I have the option of increasing the B+.
The transformer that was in the amp when I got it was a Magnetic Components 40-18033. It failed but before it did I measured the voltage at 361 VAC. There is no other HT tap on that transformer.
I suggested the Merren because of past good experience with the transformers Chris builds. He is also not as pricey as Mercury Magnetics..$270, still not cheap at $200 U S and about twice the cost of Magnetic Components.
At the time all this was going on I was just trying to replace the transformer with one of a voltage that matched the one that failed and have the option to increase the voltage.
As the owner is happy with the sound the amp is producing I thought it best to leave well enough alone.
Reducing the 180K resistor is a good idea. Thanks for the tip.
This amp has F&T 16/16 can cap which I jumped so I have 32 uf after the choke. The schematic I have indicates 32 uf on both sides of the choke.
I use and like F&T axial caps but am not fond of their can caps. I find the values all over the place. In fact I don't like can caps in any event. I am not sure anyone makes a really good cap with two caps in the same can. They must be hard to make.
I will install grid resistors and report back what I find.
I also got a bit side tracked designing a LED pilot light off the heater tap..lol
I did the best job I could with the lead dressing and as a result of that and a grounding change I made the amp has Zero no signal noise at full volume. I was a little concerned that this could be a noisy amp so I am really happy with that outcome.
I also saw the V2a/V2b issue on the schematic. I don't remember just how this build was connected after V2. The grids on V2 are jumped.
Any idea what the point of not decuppling V2 was about?
Cheers,
Billy
NOTE: I am going fishing tonight...need a break!!...lol..I will post photos of the fish when I get back.
Yes, the Merren transformer has a second set of HT taps. I measured the voltage at 436 VAC unloaded. So for sure I have the option of increasing the B+.
The transformer that was in the amp when I got it was a Magnetic Components 40-18033. It failed but before it did I measured the voltage at 361 VAC. There is no other HT tap on that transformer.
I suggested the Merren because of past good experience with the transformers Chris builds. He is also not as pricey as Mercury Magnetics..$270, still not cheap at $200 U S and about twice the cost of Magnetic Components.
At the time all this was going on I was just trying to replace the transformer with one of a voltage that matched the one that failed and have the option to increase the voltage.
As the owner is happy with the sound the amp is producing I thought it best to leave well enough alone.
Reducing the 180K resistor is a good idea. Thanks for the tip.
This amp has F&T 16/16 can cap which I jumped so I have 32 uf after the choke. The schematic I have indicates 32 uf on both sides of the choke.
I use and like F&T axial caps but am not fond of their can caps. I find the values all over the place. In fact I don't like can caps in any event. I am not sure anyone makes a really good cap with two caps in the same can. They must be hard to make.
I will install grid resistors and report back what I find.
I also got a bit side tracked designing a LED pilot light off the heater tap..lol
I did the best job I could with the lead dressing and as a result of that and a grounding change I made the amp has Zero no signal noise at full volume. I was a little concerned that this could be a noisy amp so I am really happy with that outcome.
I also saw the V2a/V2b issue on the schematic. I don't remember just how this build was connected after V2. The grids on V2 are jumped.
Any idea what the point of not decuppling V2 was about?
Cheers,
Billy
NOTE: I am going fishing tonight...need a break!!...lol..I will post photos of the fish when I get back.
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Ah yes, V2a/b, not uncommon, a lot of Marshalls, vox's, some fender's etc use these. The 1st triode is simply a gain stage, however its anode is directly coupled to the grid of the 2nd triode, which is set up as a cathode follower (acting as an impedance buffer) with the anode connected directly to the B+ node supplying them & the load in the cathode. No problems arise from directly coupling the 1st triode to the 2nd, however, as the load I'd in the cathode of the cathode follower, its cathode sits at an elevated +DC potential. This can shorten the life of "some" new production tubes that don't have good heater to cathode rating, though with most its no problem. IIRC, Merlin gives a good description of this setup on his site "Valve Wizard, as well as directions for bootstrapping of. Hope the fishing is good. Cheers
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