FX loops coming to Metro.

[UsrName]

Greetings!

I've been lurking for awhile and finally took the plunge and ordered a zero loss fx loop for my 4010 combo. It's an '83 vertical input that was modded for EL34s, an fx loop (disconnected), and a push/pull boost on the preamp pot (it was this way when I bought it). Anyway, I measured for the decoupling resistor but the B+ voltage at the PI supply node is 490v, which is higher than any values given in the instructions. So, not quite sure what to do now. I measured with the amp powered up and rocker switches to the on position and tubes in but no speakers hooked up. Does the amp need to have a speaker load when measuring?

I tried searching for a similar problem but couln't find anything. Has anyone got any idea what the problem could be? Would the mods have something to do with it? I've got a basic electronics understanding/experience but I'm kinda new to amp modding, so I'm not quite sure how to troubleshoot this problem.

Any help would be greatly appreciated.

Thanks.

Jamie

[syscokid]

...I measured for the decoupling resistor but the B+ voltage at the PI supply node is 490v...

That doesn't seem right.

You should always have a speaker-load hooked up when a tube amp is fully powered on.

If this pic of this board is similar to yours, is this where the measurement was taken?

jcm.JPG (76.08 KiB) Viewed 4134 times

[SDM]

Very glad you like the loop syscokid, also glad you found the 3 in 1 tone stack mod useful. You are correct also that UsrName's situation indeed doesn't seem right, and that one should always have a load connected to the amp when it is fired up.

To Usrname, though you should always have had a load hooked up to the speaker jack/s when amp is powered up, that wouldn't account for the high B+ reading at the PI node (assuming you are actually measuring the PI node). If you have a board like syscokid posted, measured where he indicated, and a stock 4010, then you would need a starting B+ of around 590V to get 490V at the PI node. So, even with some preamp mods, the reading you get is still way way higher than what should be possible. Could be a mod done poorly (screen to PI decoupling resistor drastically lowered/removed), rewiring during the modding throwing you off amongst various other possibilities. If you can post a gut shot of the amp, would help to see if something like that is the case.

Meantime, assuming you still have the stock PCB like syscokid posted, do be sure you measured where he indicated, and that ALL tubes are in the amp (preferably with power tubes biased properly too). For good measure, be sure your meter batteries are good, and you are properly measuring for high DC voltage with your meter.

If your amp has its PCB replaced with a turret board or such, let us know (though again pictures would cover that). Also has the PT ever been replaced? Is the amp seemingly in working order now?

[UsrName]

Got it! It's reading 407v now. I was measuring the wrong side of the 10k resistor, do'h! Thanks for posting that pic Syscokid, really helped to see it on a pcb. And just so you know, I hooked up a speaker load while powering on this time.

Thanks for the help guys, much appreciated!

[Jsinca]

Hello,

Sorry if this has been answered already in this thread. I couldn't find it.
I have a metro JTM 45 kit I built a few years ago. I installed a Vari Watt FX attenuator on the amp.

http://www.skipzcircuits.com/VariWattFX.html" onclick="window.open(this.href);return false;

My question is, will the zero loss FX unit work with this style attenuator? It varies the B+ So I guess my question is what voltage range will this loop work in?

With the attenuator off I have 400 volts on B+

Thanks
John

[UsrName]

Hello,

Sorry if this has been answered already in this thread. I couldn't find it.
I have a metro JTM 45 kit I built a few years ago. I installed a Vari Watt FX attenuator on the amp.

http://www.skipzcircuits.com/VariWattFX.html" onclick="window.open(this.href);return false;

My question is, will the zero loss FX unit work with this style attenuator? It varies the B+ So I guess my question is what voltage range will this loop work in?

With the attenuator off I have 400 volts on B+

Thanks
John

+1

I'd like to know this as well. That's the next mod I was planning after the loop.

[Jsinca]

Bump

[SDM]

The current the loop draws is much more consistent with voltage/B+ changes than the preamp/PI tubes are. At lower and lower B+, these tubes will draw less and less current. The loop will draw relatively the same amount of current (until B+ is very very low), hence its running voltages will drop faster relative to the tubes. So, at some point, the loop is just going to run out of headroom/voltage to work with.

When that will happen depends on things like the particular amp circuit, loop decoupling resistor value, starting B+. In an "average" setup, scaling the whole amp down to around half B+ is (very roughly) about when the loop may start to run out of headroom. Again though this will vary depending on the entire amp/setup. Preamp plate voltages should typically be pretty low by then, 50-80V range likely, so the tubes will be rather "starved" at this point in many cases.

So, for very very rough limits here, if scaling the whole amp and loop, the loop should function as normal until somewhere around half B+ (but again this will vary amp to amp, can't give a hard fast rule here). Below that, the loop will not have headroom to pass the signal cleanly anymore. As this translates to the loop board itself, when its board B+ reaches somewhere around say 160V-170V you may note it is starting to run out of headroom, but this too will vary depending on the amp.

So, if you use heavy scaling of the entire amp, then the ZL loop will not work out so well for you. If the loop is outside of the scaling circuit though (highly preferable), or you don't scale the amp/loop low enough to leave it without sufficient headroom (somewhere around 160V-170V on the loop board itself typically) then you shouldn't have issues.

Wish I could give more specific low B+ range limits, but since loop installs vary a good deal amp to amp, all I can do is give the very rough "average" limits as above. Any particular amp may or may not fall inside those limits, but this is the best general answer I can give here. Power scaling of the loop itself was not a consideration in its design at all, it was not designed to be scaled. May by chance happen to work out for a particular amp/user, very well may not, so can give no guarantees with scaling. Thus, in the end here, I just can't recommend scaling the loop.

[UsrName]

Hello,

Sorry if this has been answered already in this thread. I couldn't find it.
I have a metro JTM 45 kit I built a few years ago. I installed a Vari Watt FX attenuator on the amp.

http://www.skipzcircuits.com/VariWattFX.html" onclick="window.open(this.href);return false;

My question is, will the zero loss FX unit work with this style attenuator? It varies the B+ So I guess my question is what voltage range will this loop work in?

With the attenuator off I have 400 volts on B+

Thanks
John

Ok John, guess you'll have to install the loop and test it out, then let us know how it sounds.

[Jsinca]

The current the loop draws is much more consistent with voltage/B+ changes than the preamp/PI tubes are. At lower and lower B+, these tubes will draw less and less current. The loop will draw relatively the same amount of current (until B+ is very very low), hence its running voltages will drop faster relative to the tubes. So, at some point, the loop is just going to run out of headroom/voltage to work with.

When that will happen depends on things like the particular amp circuit, loop decoupling resistor value, starting B+. In an "average" setup, scaling the whole amp down to around half B+ is (very roughly) about when the loop may start to run out of headroom. Again though this will vary depending on the entire amp/setup. Preamp plate voltages should typically be pretty low by then, 50-80V range likely, so the tubes will be rather "starved" at this point in many cases.

So, for very very rough limits here, if scaling the whole amp and loop, the loop should function as normal until somewhere around half B+ (but again this will vary amp to amp, can't give a hard fast rule here). Below that, the loop will not have headroom to pass the signal cleanly anymore. As this translates to the loop board itself, when its board B+ reaches somewhere around say 160V-170V you may note it is starting to run out of headroom, but this too will vary depending on the amp.

So, if you use heavy scaling of the entire amp, then the ZL loop will not work out so well for you. If the loop is outside of the scaling circuit though (highly preferable), or you don't scale the amp/loop low enough to leave it without sufficient headroom (somewhere around 160V-170V on the loop board itself typically) then you shouldn't have issues.

Wish I could give more specific low B+ range limits, but since loop installs vary a good deal amp to amp, all I can do is give the very rough "average" limits as above. Any particular amp may or may not fall inside those limits, but this is the best general answer I can give here. Power scaling of the loop itself was not a consideration in its design at all, it was not designed to be scaled. May by chance happen to work out for a particular amp/user, very well may not, so can give no guarantees with scaling. Thus, in the end here, I just can't recommend scaling the loop.

Thank you very much for your concise reply. You've answered my questions and concerns.

I don't use power scaling below 50%. Mostly because the amp cleans up to much, and live, on stage, I'm usually at 75%. I'll purchase the loop and let you know how I make out. If it fails to work for my application, I may consider scaling just the power amp. Due to limited space on the chassis, I would rather not add a master volume to the chassis,which is almost mandatory when power scaling only the power amp.

Thanks again

[Jsinca]

Ok John, guess you'll have to install the loop and test it out, then let us know how it sounds.

I'll be the guinea pig. squeek squeek

[Jsinca]

I received and installed the loop.

Steve, just as you predicted, the loop cuts off at around 50%. Its sounds fine down to around 60%, then starts loosing headroom. For live situations this should work fine for me. I play at 100% for clean tones and 75% dirty.

One other question if you please.
The Scaling circuit has a constant B+ out. Used to feed the preamp and PI when you only want to scale the power section. Since the loop is set for 0 gain. Could I run the loop at full B+ while scaling the rest of the amp? Im thinking that since the level going into the loop would drop as the B+ drops and the loop is set at 0 gain it would just pass the lower signal on to the PI?

Thanks again
John

PS: This loop sounds incredible I finally got to hear what the El Capistan is capable of.

[erigm]

One other question if you please.
The Scaling circuit has a constant B+ out. Used to feed the preamp and PI when you only want to scale the power section. Since the loop is set for 0 gain. Could I run the loop at full B+ while scaling the rest of the amp? Im thinking that since the level going into the loop would drop as the B+ drops and the loop is set at 0 gain it would just pass the lower signal on to the PI?

Good thinking. That's what I'd try too. Are you currently scaling the whole amp, or just the power section? I assume the whole amp since the loop is currently being scaled.

[Jsinca]

Good thinking. That's what I'd try too. Are you currently scaling the whole amp, or just the power section? I assume the whole amp since the loop is currently being scaled.

Yeah the whole amp.

[erigm]

Good thinking. That's what I'd try too. Are you currently scaling the whole amp, or just the power section? I assume the whole amp since the loop is currently being scaled.

Yeah the whole amp.

So you could also just power scale the power tubes and PI, and leave the loop and preamp unscaled. I suppose that's an option. Just typing out loud.