Dark half-power Hoffman AC30

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Hi folks,


I've got a 2x el84 Hoffman AC30 that performs really well, but is kinda dark. Compared to the blackface Bassman I've got here (which is very bright)  it sounds like it has a blanket over it. I've built a couple of these before, as well as the full size version, and it seems like the 2x el84 amp is always a bit darker, perhaps due to having less headroom, but this one is just darker than I'd like. The full power AC30 I have is as bright as anyone could ever want.


It's basically standard according to the Hoffman version of the schematic other than some larger filter caps in the preamp section of the power rail, and voltages are in line with previous builds I've done (~300v on el84 plates; ~140v on plates of first 12AX7), and I've juggled coupling caps and bypass caps in the preamp with no satisfactory change. I'm wondering if making some changes in the tone stack would get the "air" and "sparkle" this amp seems to be missing.


Attached is the tone stack in question. I've played with a tone stack calculator a bit, and it seems like raising R16 (to maybe 27k?) and/or lowering the value of the treble pot R17 (maybe 250k?) as well as raising the value of the 47pf tone cap to around 100pf could essentially raise the eq shelf from 10k up a few dB. Does anyone have experience with this and do these seem like sensible or effective changes to make? I'm hoping not to make this into a different amp, just need more of that Vox shine at the top.


Also, I'm wondering what changing the value of the cathode follower resistor (V2-B) would do as this is not included in the tone stack calculator I'm using. Vox (and Hoffman) use a 56k but I notice that both 5F6A and Marshall 1987 use a 100k here. What would changing up to 100k do to the top end in a Vox?


Thanks in advance everyone
Dave in Japan

[stratomaster]

If the half power version is significantly darker than your full power one then I'd look elsewhere for a problem, not at the tonestack for a tonal tweak. 

Once you've ruled out a problem like an incorrect value (say a gridstopper accidentally 10x larger than intended) or a signal short (especially something like the carbon layer of a shielded cable contacting the signal line) then start looking at tweaks.  The one I'd try is not either resistor but the 100k. Bump that up to 120-150k for less lows and more highs.

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Hi Stratomaster,


Thanks for your reply. Never wrong to check basics, but I don't think there's any major circuit errors in this one as I built it nearly ten years ago and I've been through it a few times.


Are you referring to R15, the 100k slope resistor? If so I'll give that a shot and report back, cheers!


Dave

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Still chasing this around.


Stratomaster's suggestion of increasing the value of the slope resistor yielded some change (I tried 150K then 220K) but not exactly what I was looking for.


I did find I'd used a 10K as the V1 input grid resistor, changed that to a more standard 33k (single input jack) and that did open the top a little (maybe).


I just did another voltage check and found that though my pre-amp voltages are right where I'd like them to be, I'm at only 80% dissipation in the power section (~10 watts/el84). What would be the effect on tone of bumping this closer to 100%?

[AlNewman]

You could share an as built schematic. 

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You could share an as built schematic.

Sorry, I don't really have the ability/tech to draw shareable schematics, but it's essentially the same as the Hoffman AC30 minus two power tubes and minus the normal channel. One 220k mixing resistor is retained before the PI as I've found eliminating that makes things too hot there, even though it doesn't mix anything.


Differences:
I have a switch between two sets of V1 cathode r/c's at 1.5k and either the shown .1 or a 25uf bypass cap.
Power tube have 1k screen grid resistors rather than the 100 ohm shown in the layout/schematic.
Trannies are a Heyboer set for an 18 watt, and I think I'm using a larger Fender-type choke (forget specs, it's been a while).
Currently a 5v4 is the rectifier, and I have a 500 ohm sag resistor and additional filter cap before the first node to get B+ down a bit and eliminate hum (amp is dead quiet).
AC voltage on the 5v4 is 290 returning 342vdc on pins 4 and 6 and el84's are seeing 284vdc which is essentially identical to my considerably brighter full-size Hoffman AC30 build.


The shared el84 cathode resistor is currently 130 ohms (actually 128 measured) and I'm thinking to try a 120 there. Might this help?

[AlNewman]

As a test try bypassing the 220k 'mixing' resistor with a low value cap, maybe 250 pF.

[SEL49]

Remove that 500Ω sag resistor to increase the B+ which should bring the power up. Decreasing the 130Ω cathode resistor will also increase the power a bit but not as much as you may think.

How are you measuring/calculating the power. Be specific.

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As a test try bypassing the 220k 'mixing' resistor with a low value cap, maybe 250 pF.

thanks, Al Newman. I have tried as much as a 500uf there, just increased harshness without really opening up the top.

[MORE_Guitar_Solos]

Remove that 500Ω sag resistor to increase the B+ which should bring the power up. Decreasing the 130Ω cathode resistor will also increase the power a bit but not as much as you may think.

How are you measuring/calculating the power. Be specific.

Thanks SEL49. I'm using the Rob Robinette calculator based on voltage drop across k resistor and plate voltage and resistor value: https://robrobinette.com/Tube_Bias_Calculator.htm#google_vignette


The issue with lack of highs predates the sag resistor and I really want to keep that as it quieted the amp considerably with no appreciable effect on the tone.


I'm gonna give the 120ohm k resistor a shot and just see what it sounds like.


Thanks, everyone.

[neskor]

check the first resistor from input to the ground
also, if you are using the shielded cable try to measure capacitance of that cable or replace that cable with single solid core wire

[pdf64]

... I don't think there's any major circuit errors in this one as I built it nearly ten years ago and I've been through it a few times. ...

My guess is there's a build error, it's staring you in plain sight, but you're just not seeing it.
You need to verify every component, every connection.
Loads of hi resolution, in focus, well lit photos might help us to spot something.

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check the first resistor from input to the ground
also, if you are using the shielded cable try to measure capacitance of that cable or replace that cable with single solid core wire

Thanks Neskor.


I did have shielded cable on the input and in the tone stack, recently removed that for a slight improvement in highs. I may be creeping up on it. I'll recheck resistance to ground on the input, thanks.

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... I don't think there's any major circuit errors in this one as I built it nearly ten years ago and I've been through it a few times. ...

My guess is there's a build error, it's staring you in plain sight, but you're just not seeing it.
You need to verify every component, every connection.
Loads of hi resolution, in focus, well lit photos might help us to spot something.

Thanks pdf64. Laziness and arrogance should not get in the way! I've got a break coming up and will get into the weeds with this thing. Cheers.

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On a (perhaps tangentially related) side note:


How does everybody feel AC-type amps should be biased? I'm aware the originals were cranked way beyond design limits, but is 100% dissipation the goal? Something slightly below that? How do you guys set up your bias on these?


I built my first one of these ~20 years ago now, and it was one of those projects that just came together right the first time, so even though I knew it was biased cold I didn't mess with it. Going for a theoretical ideal with this one. I'm at 85% right now (bias-wise).


Thanks

[Jennings]

I like mine biased towards hot, but they’re nice and more mellow lower.


Just curious, but you might find it’s more about speaker choice…what speaker(s) are you playing the amp through. Personally I like my AC circuits through higher efficiency speakers. Things like Celestion alnicos or neo, V30 if you go ceramic. Brings out the snap and highs perfectly to my ears. I find them missing that aspect when I use G12 ceramics. I also find Jensen alnicos pair really nicely with AC circuits for a drier, woodier tone…which in effect gives more snap, twang and sparkle to my ears.

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I like mine biased towards hot, but they’re nice and more mellow lower.


Just curious, but you might find it’s more about speaker choice…what speaker(s) are you playing the amp through. Personally I like my AC circuits through higher efficiency speakers. Things like Celestion alnicos or neo, V30 if you go ceramic. Brings out the snap and highs perfectly to my ears. I find them missing that aspect when I use G12 ceramics. I also find Jensen alnicos pair really nicely with AC circuits for a drier, woodier tone…which in effect gives more snap, twang and sparkle to my ears.

Currently this amp is running through a 12" Eminence Red Fang, which is a discontinued alnico speaker meant to be something like a higher watt Celestion Blue. Have also run it through a Weber 1230. Neither are considered dark speakers, open back cab in both cases.


Amp is now biased to ~95%. No real change in the tone, thinking I'm just gonna live with it. 

[Big_Mike]

I am going to start a build of an AC15 based on the Hoffman AC30 layout.  Are there any component changes that need to be made when reducing the output to two EL84s?  I want to have the normal and topboost channel as drawn.  I purchased an AC15 power transformer, choke, and an AC15 output transformer.

[SEL49]

I am going to start a build of an AC15 based on the Hoffman AC30 layout.

This may have some interest for you...

     https://sluckeyamps.com/misc/Vox_15W_Double_Shot.pdf

[Big_Mike]

Interesting, thanks.  I am trying to figure out if to build an AC15 using the AC30 layout I would just need to remove two power tubes and keep the rest of the amp layout intact.

Thanks for the Sluckey dual AC15 layout, it has the top boost channel and an EF86 channel.  I don't need to the EF86 channel, just top boost and normal. 

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It should be pretty straightforward, but of course you'll need to roughly double the value of the power tubes cathode resistor for reasonable bias. I think I read your initial post and you are going with a ss rectifier, is that right? If so you might run into excessive voltages unless you've sourced a lower output PT.

[Big_Mike]

It should be pretty straightforward, but of course you'll need to roughly double the value of the power tubes cathode resistor for reasonable bias. I think I read your initial post and you are going with a ss rectifier, is that right? If so you might run into excessive voltages unless you've sourced a lower output PT.

I am actually going with a tube rectifier like on the Hoffman AC30 layout.  So the only change that has to be made to convert the Hoffman AC30 layout to an AC15 is obviously removing two of the output tubes and increasing the cathode resistor values to the two EL84s?

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Yeah, pretty much. I also removed the normal channel which necessitated a couple more changes, but you should be good with just chopping the power section in half and adjusting bias to suit. What transformers are you using? I would recommend upping the screens from 100 ohm to 1k-ish for safety and longevity.


Edit: I use this calculator for help in setting power tube bias, mine is currently at ~96% dissipation: https://robrobinette.com/Tube_Bias_Calculator.htm#google_vignette

[Big_Mike]

Yeah, pretty much. I also removed the normal channel which necessitated a couple more changes, but you should be good with just chopping the power section in half and adjusting bias to suit. What transformers are you using? I would recommend upping the screens from 100 ohm to 1k-ish for safety and longevity.


Edit: I use this calculator for help in setting power tube bias, mine is currently at ~96% dissipation: https://robrobinette.com/Tube_Bias_Calculator.htm#google_vignette

I have a Hammond 290px as the power transformer, which is the PT for vintage AC15s.  I bought the AC15 choke and output transformer from Mojotone.