AB763 Deluxe Reverb - Change Capacitors to Brighten Amp?

[davegardner0]

Last year I built myself a Deluxe Reverb using the Hoffman AB763 kit, as described in this thread:
http://el34world.com/Forum/index.php?topic=20167

The amp works great! It seems to only have two anomalies, one of which I'd like to try to fix. The first is a tiny issue, the vibrato channel's volume doesn't come on until "2", then gets loud kind of quickly. No big deal really. The second issue, though, is that the amp sounds too "bassy" to my ears. For instance, I have a strat that's extremely icepicky shrill - it sounds balanced and great though this amp while it's terrible on my other amps (Marshall JVM210, Traynor YBA1mod1, Peavey Red Stripe Bandit). Of course the rest of my guitars sound like they have too much bass/low mids through the amp...

I suspect this excess bass is also causing the amp to have a weird overtone when cranked up. See the video below... The amp makes this overtone when picking hard, and it does it when played loud AND with an attenuator. It also still does it when using an external speaker cab only.

So, is there a capacitor or set of capacitors I can change in the amp to brighten things up some / remove some bass? And, is this sort of sound typical for these amps? Mine was originally a PCB reissue made by Fender, and I remember it being a lot brighter with better distortion back then.

Amp's strange overtone with distortion:
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Here's what the distortion sounded like with the PCBs, before I rewired the amp:
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[Raybob]

I would suspect something is not right in the circuit and start with voltage readings of all tubes.  You should be able to turn volume knob to 1.5~2 and get something.

[shooter]

The volume pot could be just the type, audio vs linear.  how many speakers you got?, start there.  As for caps, I typically start with V1 Cathode and just remove whatever cap is there, play, did it go in the right direction?, to much, add 1/2 the value, play.  ONLY 1 change at a time!, when you've got that dialed, move to V2.

is it bassy both with and without verb?
Is it bassy clean and distorted?

[Tony Bones]

Only asking this because it's the kind of thing I would do: did you get the bass and volume pots mixed up when you built the amp? A 1M bass pot would be, well, bassy.

[frankenxtein]

C24 C25 .1uf capacitors page 6
http://el34world.com/Hoffman/files/Hoffman_AB763_2.pdf

Changing those last .1uf coupling caps to .015uf made a big difference in my amp.
 I just had those values handy.

 You could use .01 , .02 , .047uf Someone correct me if that's wrong please.

I should have said it Brightened up the amp

[davegardner0]

Hi Guys, sorry for some reason the email notification of replies on this thread didn't work 

Anyway I will try out some of your suggestions! I'm fairly certain the volume pot is the right resistance value. However I did but a different one that is listed on the AB763 kit page on this website as I wanted a push-pull bright switch on the vibrato channel. I bought the CTS pot with push-pull switch on this page: http://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=PotsKnobs&ORDER_ID=!ORDERID!. Now that I look at it, the page doesn't specify the taper and neither does the schematic linked above. So maybe that's the issue!! What taper should the volume pot be?

is it bassy both with and without verb?
Is it bassy clean and distorted?

It's bassy regardless of whether or not the reverb is switched on. Also it's bassy at any volume (so both clean and distorted) but it only does the farty overtone thing when really cranked.

BUT it doesn't sound as bassy and doesn't do the farty thing on the normal channel. So does this mean I need to change something in the channel 2 preamp? Or is the normal channel just usually less bassy?

[sluckey]

Now that I look at it, the page doesn't specify the taper and neither does the schematic linked above.

Sure they do. They both specify 1ma. That means 1MΩ value and Audio taper. You got the right pot.

To reduce bass make these changes...

     Change all cathode bypass caps to 2.2 or 4.7µF.
     Change C15, C24, C25 to .047µF.
     Change C22 to .001µF.

[davegardner0]

Now that I look at it, the page doesn't specify the taper and neither does the schematic linked above.

Sure they do. They both specify 1ma. That means 1MΩ value and Audio taper. You got the right pot.

To reduce bass make these changes...

     Change all cathode bypass caps to 2.2 or 4.7µF.
     Change C15, C24, C25 to .047µF.
     Change C22 to .001µF.

Oh my mistake, thanks for clarifying! Do you know if the cts spst pot sold in the store here is also audio taper?

[sluckey]

Do you know if the cts spst pot sold in the store here is also audio taper?

You already have the only CTS switch pot that I see.

[davegardner0]

Do you know if the cts spst pot sold in the store here is also audio taper?

You already have the only CTS switch pot that I see.

Right, I have that one and I was wondering if it's an audio taper, but now I see that it's 1Ma so it is!

[davegardner0]

I've been working on my amp some today and so I have some new info.

I checked out the CTS SPST 1Ma volume pot by unsoldering 2 legs then mapping the resistance vs knob angle. Turns out the pot didn't change resistance at all until it was on 3. So I replaced it with a regular 1Ma pot (no switch) that I had laying around and the amps volume taper is fixed. Yay!

Now as far as the ratty distortion / blocking distortion, I have a few observations:

-I noticed that the problem is better but not fixed when I disconnect the reverb tank from the amp. (less ratty-sounding blocking distortion)

-I tried swapping tubes, no change in the sound.

-I tried lifting the negative feedback loop, wondering if I have my OT primaries reversed. No change in the blocking distotion.

-I noticed that on the Hoffman schematic, the C22 cap is 0.01uF, whereas on the fender DRRI schematic (http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-65-Deluxe-Reverb-Schematic.pdf), the same cap which is labeled C25 is 0.001uF. So I was suspicious that this is where my extra bass is coming from. I replaced the cap with a ceramic 0.001uF I had on hand and the amp does seem a lot brighter (which I'm happy about). However I still am getting the same ratty distortion.

So any ideas what I could check next? I could follow the above suggestion and change a few more cap values but I don't want to just "treat the symptoms" if there's still an error somewhere in this amp. To anybody here who has built the same Hoffman AB763 (this design specifically with the modified tremolo, etc.) could you comment on what the distortion is supposed to sound like? At this point it's hard for me to know whether or not this amp is sounding like it's designed to sound or not...

Last but not least, while looking over the schematics I noticed that the spot where the reverb footswitch is located is different between the Hoffman AB763 and the Fender schematic. The fender footswitch's location would be between C19 and R36 on the Hoffman schematic. I'm asking mostly out of curiosity, does this change things at all?

Thanks!

[davegardner0]

One more random question for you guys, what's the recommended bias for the Hoffman AB763 deluxe? I'm wondering if the bias -varying tremolo used here means that the bias needs to be set differently than a normal deluxe reverb.

[davegardner0]

Well I did some further swapping of coupling caps and I have the amp sounding a lot better to me. I made the following swaps:
C15 to 0.047 uF
C22 to 0.001 uF
C24 and C25 to 0.047 uF

I also swapped out the two filter capacitors in parallel in the power supply from 14 uF to 22 uF, so I have 44 uF in the first stage now instead of 32 uF.

I tried also swapping out the cathode bypass caps to 4.7 uF but I undid this change as it actually increased the buzzy blocking distortion and also in my opinion made the amp kind of sterile.

I'm really happy with how things sound now!! The amp is just on the edge of farting out / getting loose but isn't crossing over which I really like. Also (not demoed here) the clean tones have become a lot more bright and snappy which is great.
! No longer available
https://www.youtube.com/watch?v=wrgLl5EbnMw

So just one follow-up question: Is it okay that I've increase the capacitance in the first stage of the power supply but left the further stages alone? Would there be any benefit to increasing those stages as well? (I noticed that the Fender Reissue amp uses 22 uF throughout for instance)
Also I may in the future try increasing the capacitance in the first stage even further if I decide there's still too much sag - how high can I go?

Thank you all for the help!! I'm thrilled with how the amp is sounding now! :)

[sluckey]

I think the way you have the caps now is fine. I've seen 60µF used for the first cap on a 5AR4 but I think that's risky. If you need to go even bigger consider using a solid state rectifier.

[DummyLoad]

I think the way you have the caps now is fine. I've seen 60µF used for the first cap on a 5AR4 but I think that's risky. If you need to go even bigger consider using a solid state rectifier.

+1

--pete

[davegardner0]

I think the way you have the caps now is fine. I've seen 60µF used for the first cap on a 5AR4 but I think that's risky. If you need to go even bigger consider using a solid state rectifier.

Ok that makes sense. Does changing the values of the other filter caps in the power supply have much of an effect on things like sag /punchiness of the amp? I was really surprised at how much of a change the increase in capacitance in the first set of caps had.

[sluckey]

Just temporarily tack solder some more caps and see if you like it. Let us know what you think.

[davegardner0]

So after playing the amp a bit I noticed some bad tones still present. Basically I'm getting a "fizzy" thin-sounding overtone when the amp is cranked. I don't know if it's blocking distortion or not but "fizz" on top of the normal guitar tone is the best way I can describe it. It seems to show up after a certain threshold volume/distortion wise.

I thought I'd see what you guys think. The strange thing is that the fizzyness seems a bit intermittent - last night I thought I had it fixed but when I turned the amp on today it was back. Then after a while the amp wasn't doing it although I'd made no change. Also I noticed that when the amp is NOT doing the fizzy thing, the fizzyness can be brought back if I turn the normal channel volume to a value OTHER than 1 or 10. Also if I pull V1 completely the fizzyness becomes even more severe.

I've been playing exclusively through the vibrato channel, including now. So I'm hoping that this interaction by the normal channel could be a clue as to what's going on??

I've tried decreasing the coupling caps even further and this didn't fix it, so now they're back to the values in my previous post. I swapped power tubes and the issue seems to be intermittent with both sets of 6V6's. I also tried changing the bias in the range of 20-28 mA and the issue seems independent of that.

It's worth noting that since my last post I also increased the other filter cap stages in the power supply. Now I'm running 3 x 16uf caps in the main stage (48 uF) and 22 uF caps in the next 3 stages. I also swapped the cathode bypass caps on V2 and V4-B to 2.2uF. Both of these changes were just to make things a little tighter sounding as in addition to the fizz the amp was a little loose sounding for me when cranked.

One last thing I noticed while playing the amp is that when I have the vib. channel volume on, say, 8 and I strum chords hard for maybe 20-30 seconds I'll notice a gradual but significant volume decrease, maybe 15% less loud. If I stop playing for a few seconds the amp's volume returns to normal when playing again. I'm playing through a Weber Mass III attenuator so maybe it's the attenuator? Can't remember having this issue when cranking much more powerful amps through it though... Otherwise maybe something is getting hot inside the amp? But what would be so stressed to heat up like that? Or is it normal for a blackface fender with its little transformers, etc.?

So any thoughts on things I can try / look into?

[dude]

You're ears can get tricked when you keep playing with the tone for a long time. The changes Sluckey suggested in one of the firsts posts should have been enough to get the mud out. Usually just changing the first 12ax7 two cathode caps from 25uf to 4.7 or 2.2 is enough. Take a  break for a day or two, then turn down the mids and bass knob.                                                                             Al

[davegardner0]

You're ears can get tricked when you keep playing with the tone for a long time. The changes Sluckey suggested in one of the firsts posts should have been enough to get the mud out. Usually just changing the first 12ax7 two cathode caps from 25uf to 4.7 or 2.2 is enough. Take a  break for a day or two, then turn down the mids and bass knob.                                                                             Al

I definitely know the effect you mean, I've been taking breaks to aviod having my ears "get used to" a certain tone for sure. It's worth noting too that I do have all of the cathode caps swapped for 2.2uF, and I have been able to get the mud out. The only issues left are the "fizz/ratty overtone" and the slight volume decrease described in my last post.

[bnwitt]

Wow.  Sluckey's changes to the stock values are quite substantial.  I would think that they would really change the character of the DR circuit.  I know lots of folks set their Bass pot way down on most AB763 amps to control the high bass output Leo put in the amps to match his single coil guitars.  It would seem that making all of these cap changes would cause bass cuts at multiple stages in the amp and I'm curious to hear from those who have done this, what happens to the setting of the bass pot after the changes are made.  5, 6 7??  I realize this is going to vary with the guitar pickup type.  Does this make the amp more usable for humbuckers?

[davegardner0]

Wow.  Sluckey's changes to the stock values are quite substantial.  I would think that they would really change the character of the DR circuit.  I know lots of folks set their Bass pot way down on most AB763 amps to control the high bass output Leo put in the amps to match his single coil guitars.  It would seem that making all of these cap changes would cause bass cuts at multiple stages in the amp and I'm curious to hear from those who have done this, what happens to the setting of the bass pot after the changes are made.  5, 6 7??  I realize this is going to vary with the guitar pickup type.  Does this make the amp more usable for humbuckers?

Well first I should point out that I'm testing the amp with a 61 SG reissue and a G&L Comanche, both of which are nowhere near a Strat. The G&L sounds like a Strat with lots more mids and mid-bass. That being said, I think that after making those changes the amp sounds a lot more like a typical Fender clean. The G&L sounds absolutely awesome clean.

As far as distorted tones go, in general I really like the sound of Gibsons / other humbucker guitars into Fender amps better than a Strat into a Fender amp. For me a Strat into a Marshall is a better pairing.  So everything I've written comes with that caveat... Of course component tolerances being 20%, using the DRRI transformers, etc adds another level of variability too.

I do realize that optimizing for humbuckers make this project a lot harder, and even still with the changes above I'm playing distorted with the bass knob on 2-3.

[Tony Bones]

Wow.  Sluckey's changes to the stock values are quite substantial. 

It's true that the cathode bypass cap and the cathode resistor (and the impedance looking up into the cathode) form a high-pass filter, but the traditional way to size the cap was to put the corner frequency WAY below any frequency of interest so that it is essentially an all-pass filter. In the case of a 25uF cap with 1.5k resistor on a 12AX7, the corner frequency is around 10Hz. Changing to 4.7uF moves the corner frequency to ~50Hz, which is just barely going to cut the lowest guitar frequencies. 2.2uF puts the the -3dB at ~100Hz which is still only going to affect the lowest octave, which is where the mud lives.

[bnwitt]

I was thinking more along the lines of the coupling cap changes rather than the bypass caps but I get what you are saying.