imperial build questions

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Bieworm
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Re: imperial build questions

Post by Bieworm »

Bieworm wrote: ↑
Mon 10/26/20 1:34 pm
The snubber is implemented.. the audible difference is not big, but it helps a little. It's not that same swirl on the decaying note we solve with the PR mod .. I think It's pretty normal with that enhanced mid bump the mid-bite pot creates. When I dial that back it disappears. I don't know yet if I don't like it.. I think I kinda do actually. Might be the honeymoon phase ... πŸ˜‰

I noticed something odd on the clean channel.. when I dime the bass it looses all bass. Beyond 3 O'clock it has no more bass. I already mentioned the volume pot is a 500k-A when it should be a 1M-A. Is there a link?

Another thing that puzzles me is that the wiper from the bass pot is connected to ground via a 10k resistor. Shouldn't the 3rd lug be that connection? On the DR it's the latter...
Edit: after looking at online pics of the imperial I noticed the bass pot is wired backwards on the schematic. So I have to move the wire from lug 1 to lug 3. Going to do that later this week when my 500V filter caps arrive, along with the 1M vol pot and the 1M-RA speed pot.

Edit #2: swapped the wire and now it works normal. Djeez a log pot is unrecognizeable when wired incorrect. The taper is so misleading...
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Re: imperial build questions

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Screenshot_20201027-123515_CamScanner.jpg
another thing I think is incorrect about the imperial schematic. It connects the screen grid to B3, which is only about 350V. It think it'd be better to downsize the resistor between B2 and B3 so I can raise that to 410V. Maybe I should try swapping R87 en R88's values so there is 5k between B2 and B3 / 10k between B3 and B4. That seems to make more sense to me.
Sad thing is I don't have any reference voltages for the imperial besides the 5E3 and BFDR voltages. the schematics of these are pretty similar to the imperial preamps. The PI is very different.
It would also make more sense to have a higher voltage on B3 for the reverb because the BF reverb has also got 410V on the plates of the send half and 170V on the platse of the recovery half.

anyway, the difference of +-80V between plate and screen on those 6V6's don't seem right...
and I think I will also try out what the effect of a bypass cap on the cathode from V1 will bring. There is none on the schematic, but maybe it's better with one? Both the 5E3 and the BFDR have one. OTOH it might be overkill for the OD channel, which shares that first preamp stage...
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Re: imperial build questions

Post by JMPGuitars »

Bieworm wrote: ↑
Tue 10/27/20 6:34 am
anyway, the difference of +-80V between plate and screen on those 6V6's don't seem right...
Did you arbitrarily decide that, or did you do research first?

What do we know about the relationship between screen and plate voltages?

What about safe voltage limits for the screens on 6v6 tubes?

Have you looked at datasheets comparing the voltage differences, especially in higher voltages such as you're using?
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Re: imperial build questions

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JMPGuitars wrote: ↑
Tue 10/27/20 9:10 am
Bieworm wrote: ↑
Tue 10/27/20 6:34 am
anyway, the difference of +-80V between plate and screen on those 6V6's don't seem right...
Did you arbitrarily decide that, or did you do research first?

What do we know about the relationship between screen and plate voltages?

What about safe voltage limits for the screens on 6v6 tubes?

Have you looked at datasheets comparing the voltage differences, especially in higher voltages such as you're using?
Not much. Just thoughts on comparison with similar amps. It's all hypothesis... not planning anything yet. The amp sounds fine as is.. going to play it for a few weeks. Maybe it's the fear of falling into the void concerning building stuff. But hey, there's still that reverb for the tremolo TMB lurking πŸ˜„
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Re: imperial build questions

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Compared the OD of the imperial with the tremolo TMB. Man that's no contest at all. The TMB is far superior.
A hypothesis... the imperial is dual channel the TMB single. Suppose I remove the components from the lead channel of the imperial and implement the components of the TMB. Adjust the voltages to the TMB voltages ofcourse. Would I be able to have the drive channel of the tremolo TMB along the clean channel of the imperial?
Important note is that I never push the power tubes into OD on my TMB , so I conclude it's the preamp OD I love so much.
The mid pot circuit can be easily mimic'd with fixed resistors, no? I always turn it to near max, so I make the compromise of not being able to tweak that anymore.
Is there some logic in this hypothesis? Am I overlooking some crucial things?
Ofcourse I will have to move stuff around to maintain the clean channel. + the imperial shares the 1st preamp triode for Josh channels, so I am aware of the possibility the voltages from imperial to tremolo TMB 1st preamp stage will have an impact on the clean channel too. Although the voltages don't differ that much...
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Re: imperial build questions

Post by JMPGuitars »

Bieworm wrote: ↑
Wed 10/28/20 3:15 pm
Compared the OD of the imperial with the tremolo TMB. Man that's no contest at all. The TMB is far superior.
A hypothesis....
I think you should make the Imperial the best Imperial it can be. Then build the Trem TMB Reverb. We both know that's what you really want, and trying to make a Chevy a Ford doesn't work. Well, I guess a Chevy could be a Ford if it was Found On Roadside Dead. πŸ€·β€β™‚οΈ

Seriously though. The Imperial sounds good, and if you don't want it, you could sell it. You don't have to keep every amp you build.

Side note: I will eventually build the 6v6 version of my Trem TMB Reverb, and then release the docs. I dunno if it will sound as good as the EL84s though.
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Re: imperial build questions

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Well.. I tinkered around with some stuff and I think it's fatter and greasier sounding.

1. I put a B500k pot in series with R21, the last resistor of the drive channel right before the input of the PI circuit. By raising the resistance I can get the OD at a little less volume.
2. I put a 47uf 63V cathode bypass cap parallel with the 820R (R3) on V1. This beefs up the tone and makes it somewhat greasier.

Is there any harm in doing those mods?
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Re: imperial build questions

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Here is a video of the imperial. Wonder if this fizz is power tube related...does it at lower voltages too..

https://www.dropbox.com/s/3j1sairhhvb8t ... 5.mp4?dl=0
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Re: imperial build questions

Post by JMPGuitars »

JMPGuitars wrote: ↑
Sun 10/25/20 2:17 pm
Take a look at #4 on my EF86 Xtra schematic for the snubber: files/JMPGuitars_18_Watt_EF86_Xtra_Schematic.pdf

I think for two 6v6 power tubes you would change the 3K to 1.5K, and the .0047uF to .01uF.
I couldn't hear the fizz, you didn't let a distorted chord ring out long enough (at least not for these headphones lol). But do what I suggested above if it's bothering you.
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Re: imperial build questions

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JMPGuitars wrote: ↑
Tue 11/03/20 2:14 pm
JMPGuitars wrote: ↑
Sun 10/25/20 2:17 pm
Take a look at #4 on my EF86 Xtra schematic for the snubber: files/JMPGuitars_18_Watt_EF86_Xtra_Schematic.pdf

I think for two 6v6 power tubes you would change the 3K to 1.5K, and the .0047uF to .01uF.
I couldn't hear the fizz, you didn't let a distorted chord ring out long enough (at least not for these headphones lol). But do what I suggested above if it's bothering you.
Not really bothering me, but gonna try that anyway. Maybe it's the mid focussed sound that is so peculiar ..have to play it in the band, but at the moment it's not allowed
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