NOS #102 Build

[dbharris]

A couple of small updates. Keep in mind this was my first ODS build.

I emailed the tube vendor asking if they had a single tube that was a closer match they could sell me. Even though I bought this quad over a year ago they immediately emailed back apologizing that the one tube should not have been part of that set. They are going to dig through their collection and see what they can come up with to make it right.

I tried an unbalanced tube in the PI with the weaker side being the feedback side. It was easier to balance out with the 10K trimmer.

My new bourns trimmers came in and I swapped in 20K which made it much easier to balance as well. With JJs back in the amp, it is all set up and sounds great. When setting the PI, I had to run signal through the normal input bc my tablet that I'm using to generate the sine wave only puts out .75v peak to peak. I noticed that the sweet spot is different depending on if you send a 500hz or 1khz wave. I settled on a 587hz signal, basically a D5 or close to middle range for a standard tuned guitar. With the PI balanced that way, the plate voltage on V3 is almost the same on both plates.

I moved the amp down to my home office/music room and connected it to the dumbleator for the first time. Wow, this is the way. Without it the amp is just a tad bright for me. But a little less sharp and easy to dial in with the dumbleator. Not using any of the various bright switches. Think I may need to build a portable smaller footprint dumbleator too.

I have only played through the EVM-12L so far. Will try the G12-65 cab tomorrow.

-Dan

[Stephen1966]

Who was the tube vendor? - we like positive reviews here!

Measuring plate voltage at V3 (your PI) is really only half the story since it doesn't adequately account for the OT or the power section. This is why we balance at the power tube cathodes. Where you have it, much depends on the whether the PI tube is balanced or not, and rather than 0V between the plates, most people around here look for a 7V difference but that's moot because it can all be for nothing unless you take the power section into account, as well.

These amps are bright. "A tad bright..." is what most players experiencing the "Dumble Sound" experience as well. There's nothing essentially cautionary about your observations except that this is what you can expect from these amps. It's the quality of the bright, higher frequencies, that are remarkable though. Ice-pick, ear-offending, no. Silky, harmonically rich, full. And with notes that readily sustain and bloom. That's where it should take you. The EVM12L is a fine speaker, I will be interested to hear what you make of it with the Celestion.

In your artistic approach, adding the Dumbleator at this stage is only adding a layer of complexity to your tuning that may ultimately make it harder to dial the amp in. My experience of Dumbleators (as well) is that they beautify the tone of the amps but you might be putting the cart before the horse here; try dialling the amp in before you process the signal through the Dumbleator, is my advice. You want a clean signal with a minimum of filtering and attenuation. Switching it over to OD with PAB afterwards, is good for gauging the equivalence of tone between clean and OD. A balanced tone, a transparent shift as you engage the second channel, is the goal.

It's an interesting observation that you found circa 600Hz is the sweet spot tuning range, You might be onto something there because I've also questioned the convention of using 1kHz as the input frequency before, as well. I don't think it matters too greatly because you will hear differences in response if you use 1kHz across your amps as well. But an interesting point nonetheless. If you are only able to put out 0.75V pk-pk though, you are restricted to injecting your signal at the input of the amp (not the PWR AMP IN) because ideally, you need to drive the power section to the onset of clipping and you won't get that if you bypass the gain stages of the preamp.

[dbharris]

LA Vintage Gear is the tube vendor. Hopefully will have an update from them soon.

Between a LP, two strats, and a lapsteel with humbuckers, all sound great. The strats don't do that fender clean thing with this tone stack but still sound nice. I could get closeish with scooping the mids and engaging the bright switch. Overdrive ends up not being as smooth though.

The G12-65 cab is nice too. More forgiving, and rounder or softer feeling than the EVM-12L. I like the bass response better with the EV, it's more snappy and even feeling. Really just a matter of different tone settings to find the sweet spot.

I am enjoying playing it and an pretty impressed with how versatile it is between using different guitars, inputs, switches, etc. Going to burn it in a bit more this week and then next week record some clips.

-Dan

[Stephen1966]

LA Vintage Gear is the tube vendor. Hopefully will have an update from them soon.

Between a LP, two strats, and a lapsteel with humbuckers, all sound great. The strats don't do that fender clean thing with this tone stack but still sound nice. I could get closeish with scooping the mids and engaging the bright switch. Overdrive ends up not being as smooth though.

The G12-65 cab is nice too. More forgiving, and rounder or softer feeling than the EVM-12L. I like the bass response better with the EV, it's more snappy and even feeling. Really just a matter of different tone settings to find the sweet spot.

I am enjoying playing it and an pretty impressed with how versatile it is between using different guitars, inputs, switches, etc. Going to burn it in a bit more this week and then next week record some clips.

-Dan

Very cool! Looking forward to hearing your clips. When you say, the "strats don't do that fender clean thing," how do mean? Are they low output pickus? (Try it with FET boost) or is it the Fender "twang" you are referring to. I mean, it sounds like a complex response you are getting. Could be pickups, the speaker, or some tuning to the amp circuit that needs to be considered. Have you set your OD trim yet? And, what did you use for the OD input? - a 220k in series with a 100k pot is typical. If I remember correctly a good start point for the trimmer is 27k to ground, or with the trimmer centre pitched towards the tube sockets. That's got to be experimented with.

The test point, using a scope and looking at the trace appears in #183 as just after the second stage of the OD coupling cap (blocking your high DC voltage) but before the resistor (180k in #183) before the line out to the Ratio pot.

183_side copy.jpg

Get that scope on the power tube cathodes though - trust me, it is life-changing

[dbharris]

LA Vintage Gear is the tube vendor. Hopefully will have an update from them soon.

Between a LP, two strats, and a lapsteel with humbuckers, all sound great. The strats don't do that fender clean thing with this tone stack but still sound nice. I could get closeish with scooping the mids and engaging the bright switch. Overdrive ends up not being as smooth though.

The G12-65 cab is nice too. More forgiving, and rounder or softer feeling than the EVM-12L. I like the bass response better with the EV, it's more snappy and even feeling. Really just a matter of different tone settings to find the sweet spot.

I am enjoying playing it and an pretty impressed with how versatile it is between using different guitars, inputs, switches, etc. Going to burn it in a bit more this week and then next week record some clips.

-Dan

Very cool! Looking forward to hearing your clips. When you say, the "strats don't do that fender clean thing," how do mean? Are they low output pickus? (Try it with FET boost) or is it the Fender "twang" you are referring to. I mean, it sounds like a complex response you are getting. Could be pickups, the speaker, or some tuning to the amp circuit that needs to be considered. Have you set your OD trim yet? And, what did you use for the OD input? - a 220k in series with a 100k pot is typical. If I remember correctly a good start point for the trimmer is 27k to ground, or with the trimmer centre pitched towards the tube sockets. That's got to be experimented with.

The test point, using a scope and looking at the trace appears in #183 as just after the second stage of the OD coupling cap (blocking your high DC voltage) but before the resistor (180k in #183) before the line out to the Ratio pot.

183_side copy.jpg

Get that scope on the power tube cathodes though - trust me, it is life-changing

The "twang" was what I meant. I can get it crystal clean it but it's not that Wind Cries Mary tone (not that I was expecting type of tone from this circuit).

I have been playing around with the OD trimmer. I have a 220K feeding a 100K trimmer. I think I left it at 26K to ground. I've tried between 22K and 38K. There is still a ton of gain on tap.

I am going to start chopsticking around my lead dress too in case I can find some bloom/liveliness that way.

I have my FET setup for the "low headroom" bias but I like it quite a bit. I believe I set it for about 6db of voltage gain compared to the normal input.

I'll try to find an economical signal tester to be able to go in through the power amp.

-Dan

[Stephen1966]

LA Vintage Gear is the tube vendor. Hopefully will have an update from them soon.

Between a LP, two strats, and a lapsteel with humbuckers, all sound great. The strats don't do that fender clean thing with this tone stack but still sound nice. I could get closeish with scooping the mids and engaging the bright switch. Overdrive ends up not being as smooth though.

The G12-65 cab is nice too. More forgiving, and rounder or softer feeling than the EVM-12L. I like the bass response better with the EV, it's more snappy and even feeling. Really just a matter of different tone settings to find the sweet spot.

I am enjoying playing it and an pretty impressed with how versatile it is between using different guitars, inputs, switches, etc. Going to burn it in a bit more this week and then next week record some clips.

-Dan

Very cool! Looking forward to hearing your clips. When you say, the "strats don't do that fender clean thing," how do mean? Are they low output pickus? (Try it with FET boost) or is it the Fender "twang" you are referring to. I mean, it sounds like a complex response you are getting. Could be pickups, the speaker, or some tuning to the amp circuit that needs to be considered. Have you set your OD trim yet? And, what did you use for the OD input? - a 220k in series with a 100k pot is typical. If I remember correctly a good start point for the trimmer is 27k to ground, or with the trimmer centre pitched towards the tube sockets. That's got to be experimented with.

The test point, using a scope and looking at the trace appears in #183 as just after the second stage of the OD coupling cap (blocking your high DC voltage) but before the resistor (180k in #183) before the line out to the Ratio pot.

183_side copy.jpg

Get that scope on the power tube cathodes though - trust me, it is life-changing

The "twang" was what I meant. I can get it crystal clean it but it's not that Wind Cries Mary tone (not that I was expecting type of tone from this circuit).

I have been playing around with the OD trimmer. I have a 220K feeding a 100K trimmer. I think I left it at 26K to ground. I've tried between 22K and 38K. There is still a ton of gain on tap.

I am going to start chopsticking around my lead dress too in case I can find some bloom/liveliness that way.

I have my FET setup for the "low headroom" bias but I like it quite a bit. I believe I set it for about 6db of voltage gain compared to the normal input.

I'll try to find an economical signal tester to be able to go in through the power amp.

-Dan

Regarding the 'twang', I've not experienced it with the amps either but then I don't have a strat to try it, just an approximation with my pickups in single coil mode. My guitar is very different in construction from a strat though, so it's a poor approximation at best. One way of approaching this though might be to take an amp that does produce the tone with the instrument and then look at what makes that amp, different from this one. What you do get with these amps, is great touch sensitivity, compression, saturation an almost hifi sound compared to other amps. And one thing you have to do with these amps, is play correctly because if you make any mistakes, they will be heard all the way to the back of the auditorium.

OD is something else. When you talk about 'gain' are you thinking of the trimmer as a voltage divider? It does work that way of course, but I haven't noticed any real difference in gain from this function and none that can't be compensated for in the LEVEL and RATIO settings. I suggest instead, you think of the trimmer as part of a RC low-pass filter instead. For me, this is where the real audible difference is made. In conjunction with the interelectrode capacitance of the tube, the Miller effect and Miller capacitance the trimmer is the most significant effect I hear, trimming the high frequencies. There are a couple of ways you can look at this, the first is the impression that it boosts the high frequencies, and the second that it attenuates the high frequencies - dependant it seems on which way you turn the trimmer. Only the second is true though, because this filter is passive and it can only attenuate; for argument's sake passive at least, though I am sure the non-linear nature of the tube operation counts as some kind of active (reactive) response. In doing so, it might give the impression it is boosting certain frequencies, but only because it is attenuating other parts of the frequency spectrum. Much like a tone pot on a Fender, wide open, it sounds like it is boosting the high frequencies but really when you dial it back you attenuate the highs which in turn, emphasises the mids and lows, the lower you go. The OD trimmer is worthy of a topic thread all of its own because it isn't just doing one thing, it's performing double/triple perhaps even quadruple duty and out of all the functions it is performing, it is the apparent boosting of the mids to lows that stands out, tangibly, and emphatically enough to be clearly audible.

An economical signal tester! Hmm, good luck with that. I guess if your scope has FFT and a bode plot function you could go with that, but by far the cheapest and the best instruments you already have, are your ears. They come in pairs apparently!

[dbharris]

I decided it was time to finish this amp. LA Vintage sent a replacement Sylvania 6L6 that was a closer match in the quad. Unfortunately though it is a different construction, tall bottle with a single halo getter on top rather than short bottle with the double side getters. I don't have much choice but to just live with it and at least the PI can be balanced with this arrangement.

I swapped the Centralab pots back into the OD section. Had to replace one of the shielded runs again...I think I am going to retire this Gepco shielded wire it is just too finicky. Set the OD trimmer at about 29K to ground. The OD is very loud compared to the clean channel. I don't have any issues balancing the tone between clean and OD (in other words the OD is a nice extension of the clean channel and amazingly touch sensitive) but I feel like it is a little hard to balance the volume between clean and OD. To get the OD tone I like the clean is always a little too quiet.

I went with IRC RN65D 221K and Roederstein MK3 150K plate resistors. With OD and PAB I get nice bloom at moderate volumes. FET input sounds really nice too. Without PAB it does not seem to bloom as readily. I will try some loud playing over the next 2 weeks when possible to see if that gives up the goods easier. Overall though, the amp has a wide variety tones and is very fun to play. Onwards to planning a 2nd Gen build in the near future.

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-Dan

[dbharris]

Final (I think) update. Pulled the Clarostat pots from the overdrive controls back out and replaced with new CTS. The Clarostat pots had an ever growing range of rotation approaching now 300 degrees. They functioned fine otherwise...but seemed like a problem would arise eventually.

I found some shielded wire in my stash I used when making custom cables for my pedal board many years ago. They are Chandler branded "low capacitance instrument cable". Very easy to use, so I went ahead and replaced 3 of the 4 shield runs on those pots that previously had the Gepco that was prone to failing.

I also backed down the OD trigger to about 24.5K wiper to ground. Added RTV to the v1 and tone stack caps.

Lastly I adjusted the amps bias just a hair cooler and readjusted the PI balance with the amp set how I like to run the master volume and presence now.

Played it for a couple hours and happy with it.

PXL_20231125_193744737.jpg

[ijedouglas]

Great job, congrats!

[Matt J]

Relay transformer is Hammond 166G6.

I have that same relay transformer I plan on using for some future builds this winter and spring. Did you tie off or ground the green/yellow center tap wire? It looks like you tied it off from the photos but I wanted to double check.
Thanks!
Matt J.

[dbharris]

Relay transformer is Hammond 166G6.

I have that same relay transformer I plan on using for some future builds this winter and spring. Did you tie off or ground the green/yellow center tap wire? It looks like you tied it off from the photos but I wanted to double check.
Thanks!
Matt J.

Hi Matt,

Yes, I trimmed it and shrink wrapped over the end of the center tap. My understanding is it keeps the switching circuit more quiet. But I was just copying what others here have done. Good luck with your build!

-Dan

[dbharris]

I've never had a tube with this blue florescence glow before. The filament glows much brighter too on that one tube. This was the replacement tube that LA Vintage sent me. I noticed because the amp sounded a little splatty under heavy distortion (much higher than I usually play) for a solo I was learning. I did a quick visual check, saw that, and shut it down.

I pulled out my meter and the bias is stable and where it should be.

Is it safe running this tube or do I need to swap it out? If I need to retire it, I will probably just order a TAD quad rather than fuss more with trying to find a single NOS tube to match the other 3.

Thanks,

Dan

[martin manning]

A faint blue/purple glow on the inside surface of the envelope is normal, and the heater brightness can depend on how much of the heater extends outside of the cathode tube. Both the heater brightness and the blue glow look excessive to me, possibly indicating incomplete vacuum ("gassy"). I'm assuming you mean that this tube biases normally, reasonably close to the others. A curve trace and heater current check might reveal more. IDK if it is dangerous to run it, but I think I would go with a new matched set and save these for spares.

[dbharris]

A faint blue/purple glow on the inside surface of the envelope is normal, and the heater brightness can depend on how much of the heater extends outside of the cathode tube. Both the heater brightness and the blue glow look excessive to me, possibly indicating incomplete vacuum ("gassy"). I'm assuming you mean that this tube biases normally, reasonably close to the others. A curve trace and heater current check might reveal more. IDK if it is dangerous to run it, but I think I would go with a new matched set and save these for spares.

Thanks, Martin. I'll do just that.

-Dan