Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bash on December 04, 2023, 09:23:58 am
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Hi everyone,
Based of Rob Robinette's and Steve Luckey's fantastic heap of information, I created the following:
-A blackface, AB763 preamp stage
-A bassman, 5F6 power stage
I also made a few additional tweaks and changes:
-Changed the mid pot to 50K, so it should give a a nearly flat eq-curve when maxed (using Tone Stack Calculator https://www.duncanamps.com/tsc/)
-Added a boost breaking the mid-to-ground (should remove the eq-section)
-Added another boost by adding a bypass after V1-B
-Adding a master volume
-Removing the standby switch (because I don't like them)
Is there any reason not to do this?
Do you guys foresee any issues that I overlooked?
Any feedback would be appreciated.
PS
I have completely updated the layout and replaced that. You can find that at the end of page 2 of this thread.
That layout is a lot easier to work on and better overall (I think).
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As drawn, the "Master Volume" is actually just balancing (or imbalancing) signal to the output tubes. Also, I assume the lower leg of that pot would be placed before the grid stopper (which is typically placed on the valve socket).
There is no trim pot in your bias circuit. Sluckey's "Marshall Style" bias circuit is the bee's knees (see http://sluckeyamps.com/misc/AB_Lite_II.pdf (http://sluckeyamps.com/misc/AB_Lite_II.pdf), but also present in many of his other amps.
I assume you're going to use 6L6GT's, right? best to let us know.
Didn't spend much time with the rest, but, I've built several amps very similar to this -- they sound great.
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I'm not sure about that Boost 2 switch. I would rather put the switch between the 270K and ground. I know that will give a very noticeable boost.
I think you will not like that crossline MV.
I would make the bias voltage adjustable, especially if using 6L6s or 5881s or some other big bottle.
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acheld is right, it is not a master volume
Build this one, IMO it is the best one;
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acheld is right, it is not a master volume
It IS a crossline master volume control. Look at some Matchless schematics.
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Thank you for all the input.
I will update the layout later today.
acheld is right, it is not a master volume
It IS a crossline master volume control. Look at some Matchless schematics.
That is what I thought as well. Simply using the volume as done by Matchless, I am just not sure whether this is the best place for it. Is there any benefit in moving to the left, directly after the phase inverter?
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Oooh, always learning a new thing in this forum! Crosssline MV new to me.
I would still move it left of the grid stoppers.
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I would still move it left of the grid stoppers.
Me too.
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The valve types aren’t noted on the schematic?
Certainly with a 12AX7 LTP, the type 3 master volume can work better at low levels with some additional resistance between the control and the LTP anodes.
So putting it to the right of the grid stoppers, as per the schematic, can help with that.
But then the grid stopper action of rejecting high frequencies is messed up.
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Thank you again for all the feedback.
I've updated the schematic in the original post.
Most of it is based around Rob Robinettes schematic now.
I've also added a balance-pot for the bias and a three position switch for the negative feedback.
A question though, Steve Luckey's schematic (the one I copied mostly) says "47 Ohm for 8 Ohm speaker, 100 Ohm for 4 Ohm speaker" for that resistor, Rob's says 47 Ohm and Mojotone says 100 Ohm.
For being able to switch between 4, 8 and 16 Ohm speakers, what value do you recommend?
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A question though, Steve Luckey's schematic (the one I copied mostly) says "47 Ohm for 8 Ohm speaker, 100 Ohm for 4 Ohm speaker" for that resistor, Rob's says 47 Ohm and Mojotone says 100 Ohm.
For being able to switch between 4, 8 and 16 Ohm speakers, what value do you recommend?
The NFB should be connected directly to a tap on the secondary, not after the selector switch. So, you would still use 47Ω for 8Ω tap or 100Ω for 4Ω tap. This way the NFB will remain constant regardless of which tap the speaker load is connected to.
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Ah... the value depends on which secondary tap you use for the negative feedback. Not on the impedance of the attached speaker...
I completely misunderstood that remark in your schematic.
Thanks again.
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Sorry for the vague note. I really should have made that clear. Crystal clear! :laugh:
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Well, with all the information you share readily with everyone, I can only be thankful.
And I am certainly glad I asked.
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What are you wanting in the sound by trying to build a black face AB763 preamp with a Bassman 5F6A power amp?
Looking at your schematic, your power amp section is a little off from a 5F6A.
The PI is considered part of the power amp and a 5F6A has a presence control tied to the PI. While BF AB763 amps do not have a presence control. Also differences of, a PI resistor value, coupling caps values, no presence control. And the FB your using is from a AB763 amp.
All combined together, this will change the power amps sound some what. So it's not a Bassman 5F6A power amp section anymore. :dontknow:
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I've updated the schematic in the original post.
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Valve types still not noted?
The point being that your type 3 master volume loading a 12AT7 LTP may not provide results that please you.
But then it’s all a good learning experience :)
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I've updated the schematic in the original post.
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Valve types still not noted?
The point being that your type 3 master volume loading a 12AT7 LTP may not provide results that please you.
But then it’s all a good learning experience :)
Sorry, I forgot to update the GIF-file. I've updated them both now.
So now they clearly show the tubes.
I will use a pair of JAN Philips 7581's that I have still lying around, instead of the 6L6's. From what I could found about them, they are interchangeable.
I got rid of the 5F6 power section and it is back to a AB763 one again (mostly based on the schematic on Rob Robinette's website).
Now mostly figuring out how to create a layout that I can build with parts that are easily available in Europe.
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What are you wanting in the sound by trying to build a black face AB763 preamp with a Bassman 5F6A power amp?
I was hoping to create an amp with the sound of a Blackface amp, without the hassle of needing to bias it.
But I've abandoned that last part.
Instead I will add access point at the back for multimeter probes, so I can bias it without the need for opening up the amp.
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I was hoping to create an amp with the sound of a Blackface amp, without the hassle of needing to bias it.
A 5F6A needs to be biased just like an AB763 amp.
Instead I will add access point at the back for multimeter probes, so I can bias it without the need for opening up the amp.
You will still need to open the amp, you need to know the dcv on the power tubes plates to set the bias. Unless you have an access point for the dc plate voltage on the outside of the chassis, but now it's getting a little (a lot) dangerous.
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I was planing to make bias test points like described here:
https://waynereno.com/Bias_Test_Points.html
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Measuring tube current via those test points is only half of the process of biasing an amp. You must also measure the plate voltage. How will you safely do that?
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On the website of my main supplier they have the following biasing guide:
https://www.tube-town.net/cms/userfiles/downloads/bias-setup-e.pdf
For measuring the plate voltage they suggest:
"Plate voltage It is best to measure the plate voltage directly on the socket of the output tube. On the common octal tubes, this can be measured with a voltmeter between pins 3 and 8. This ensures that the real plate voltage is determined, independent of any influence from other components.
The easiest method is to remove the tube and insert the meter probes in the socket. The meter should be set to measure DC voltages. In normal operations, with tubes, the real voltage under load will be somewhat less than at idle; if 450 volts unloaded is are measured, as a rule the loaded voltage will be between 420 and 430 volts.
These values shouldn't be viewed as written in stone, as the voltage from the wall can vary from region to region and from day to day."
So, that's how I was planning on doing it.
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The easiest method is to remove the tube and insert the meter probes in the socket. The meter should be set to measure DC voltages. In normal operations, with tubes, the real voltage under load will be somewhat less than at idle; if 450 volts unloaded is are measured, as a rule the loaded voltage will be between 420 and 430 volts.
So, that's how I was planning on doing it.
That's BULLSHIT! May as well just take a wild ass guess.
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Ah.... at least it is good to hear that that method makes no sense, before I try it.
In that case I better build the cabinet in such a way that it is easy to get the amp out, to do it the regular way.
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The easiest method is to remove the tube and insert the meter probes in the socket. The meter should be set to measure DC voltages. In normal operations, with tubes, the real voltage under load will be somewhat less than at idle; if 450 volts unloaded is are measured, as a rule the loaded voltage will be between 420 and 430 volts.
So, that's how I was planning on doing it.
That's BULLSHIT! May as well just take a wild ass guess.
I don’t see it like that.
For a given amp at a given mains voltage and a given cathode current, the HT or output valve anode voltages are also a given.
The only variable is the mains voltage. That’s simple enough to check. Variac it back to nominal if it’s more than a % or 2 off.
I was planing to make bias test points like described here:
https://waynereno.com/Bias_Test_Points.html
A 3.5mm stereo mini jack socket is much easier, just needs a 6mm mounting hole drilling somewhere convenient.
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May as well just take a wild ass guess.
I don’t see it like that. For a given amp at a given mains voltage and a given cathode current, the HT or output valve anode voltages are also a given.
The only variable is the mains voltage. That’s simple enough to check. Variac it back to nominal if it’s more than a % or 2 off.
I don't see how it's a given.
With any different power tube, brand/type, (even staying in, say, the 6L6 family) and depending on the PT, as you increase the power tubes current draw, the dcv will drop. But it will drop at a different rate and value with the different PT and tubes.
No way to tell without checking the tube plate voltage.
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In that case I better build the cabinet in such a way that it is easy to get the amp out, to do it the regular way.
You can still use the test points and 1 ohm resistor in series with the power tube cathode with the test points you were going to use, that's fine. They are nice for holding the meter probs while you adjust the -bias.
But you will also need to pull the chassis to measure the power tubes plate dcv.
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May as well just take a wild ass guess.
I don’t see it like that. For a given amp at a given mains voltage and a given cathode current, the HT or output valve anode voltages are also a given.
The only variable is the mains voltage. That’s simple enough to check. Variac it back to nominal if it’s more than a % or 2 off.
I don't see how it's a given.
With any different power tube, brand/type, (even staying in, say, the 6L6 family) and depending on the PT, as you increase the power tubes current draw, the dcv will drop.
Indeed :thumbsup:
On a given amp chassis with a given mains voltage, a given current draw produces a given voltage.
But it will drop at a different rate and value with the different PT
Bash is just looking to swap output valves, not the mains transformer.
and tubes.
The idea here is to adjust the bias to achieve a suitable idle current. That’s the given current I mentioned previously.
Hence Ohm’s law dictates the resulting HT and anode voltages.
As it’s a static scenario, at idle, the valves are effectively just voltage controlled current sources, the bias trimmer being the voltage controller.
What mechanism could cause HT / anode voltage to be different when swapping between one current source and another, given that we’re setting them for a given current?
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Thanks again, everyone, for the valuable input.
I could just create 5 multimeter inserts, like in this image (of course making sure that they are completely isolated from chassis).
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There’s no need / benefit in that;
A/ lead dress is a big thing, output valve anode wiring is a super powerful electrical field transmitter. That can couple back to earlier stages and cause instability.
B/ the anode V DC of both output valves will be virtually identical.
C/ the anode V DC of either / both output valves will be virtually identical to the HT V DC node, eg the reservoir cap +ve terminal, OT CT.
D/ high voltage terminals must not be exposed to the user, eg consider if you were fiddling with something at the back of your amp and a stray guitar string poked into the socket.
E/ as explained previously, for a given current draw, at idle these voltages can only vary in direct proportion to the mains voltage.
So if mains voltage is close to nominal and the idle current is set to the desired level, the HT / anode voltage must be a particular value, ie whatever it was when the amp was first built and you determined what the appropriate idle dissipation and current should be (provided that your mains voltage was close to nominal at that time).
Edit - the topic of needing to check the anode voltage every time bias is adjusted was discussed to death over at https://www.thegearpage.net/board/index.php?threads/biasing-an-evh-5150-is-it-really-this-easy.2088033
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Well, that simplifies the layout a little.
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""....remove the tube and insert the meter probes in the socket....""
That's (nearly) UN-loaded voltage. (The power tubes are 90+% of the supply current.)
We actually want loaded voltage.
Some supplies sag 5%, others sag 30%.
If the supply is saggy, this method is VERY conservative. You may "calculate" 12W Pdiss and really run at 6W Pdiss.
But you know, in the old days we never stressed "bias" unless the tubes melted.
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Made a few minor changes in the schematic.
I intend to use the Hammond 290CEX power transformer (as the ones mentioned in Steve's and Rob's schematics are next to impossible to get in the Netherlands).
Given that it is a direct replacement for the Vibrolux, I assume that it is also suitable for this design.
Is there anything that I need to compensate for, since it only supplies 325-0-325 Volts instead of the 355 - 0 - 355 in Rob's design?
Like changing the 22uF filtering Elco's to 16uF, like in the original Vibrolux schematic?
BTW
I looked everywhere, including Igor Popovich book, but could not find a good description and/or calculation examples explaining what the ideal value for these are.
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Is there anything that I need to compensate for
I wouldn't change anything.
I looked everywhere, including Igor Popovich book, but could not find a good description and/or calculation examples explaining what the ideal value for these are.
What are you talking about?
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To get plate voltage without taking out chassis, don’t pull the power tube out all
the way, then check voltage..? Not the safest thing to do, but maybe a solution.
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I can recommend the 290DEX. Right now I'm using this PT in a 5F6a-ish build.
With 6L6GC:s at ~60% idle dissipation I have 400V on the plates (using a 5U4GB). Sounds nice and compresses real nice when pushed a little.
With 5AR4 plate voltage is roughly 440V and a bit stiffer/tighter.
/Max
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I wouldn't change anything.
Thanks, that is all the information I need for now
What are you talking about?
On schematics for the AB763 I see many different values for these, varying from 10uF up to 47uF, with anything inbetween.. I am assuming there is a reason to go for a different value. Just can't find that reason.
I can recommend the 290DEX. Right now I'm using this PT in a 5F6a-ish build
Thank you.
I was looking at that one too, but but that one has the same almost the exact same specifications as the 290CEX (apart from a slightly higher max current at the secondary of 230 mA instead of 207 mA).
Furthermore it 30% more expensive and it seems to be on backorder everywhere in Europe.
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I've added my intended layout to the original post.
Was a bit of a puzzle, trying to keep most signal-bearing cables as short as possible and ending up with a front panel that looks simple and tidy.
The green resistors will be low-noise RN65D 1/2 Watt ones.
For the rest I will use 2 Watt ones (unless they're 5 Watt). I just like the size and look of these.
The negative feedback switch will be a On-Off-On switch, but I had to change the resistor values a bit for it to work the same (and I used two 1k resistors in parallel instead of one 680 Ohm, because I can't get that one in a low noise version).
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The resistors on the NFB switch are connected to the ground lug of that terminal strip. This means that you have NO NFB regardless of which position the switch is in. May want to move those resistors to an ungrounded lug.
Bright switch is missing a bright cap.
Remove the jumper between V3 and V4 pins 8.
Remove the short black jumper between the balance and bias pots.
I didn't look at anything else.
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Thanks for the imput.
The resistors on the NFB switch are connected to the ground lug of that terminal strip. This means that you have NO NFB regardless of which position the switch is in. May want to move those resistors to an ungrounded lug.
That is the idea. You have three flavours of NTB; 820 Ohm, 470 Ohm or 1,5 K.
I will have a look at the rest. Thank you.
Still need to carefully check everything myself again.
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Thanks for the imput.
The resistors on the NFB switch are connected to the ground lug of that terminal strip. This means that you have NO NFB regardless of which position the switch is in. May want to move those resistors to an ungrounded lug.
That is the idea. You have three flavours of NTB; 820 Ohm, 470 Ohm or 1,5 K.
You're missing my point. The center lug of that terminal strip is connected to chassis ground via the mounting screw and nut. This also puts a ground on the blue wire going to the board. This ground kills all NFB. My point... don't use the center lug. Move the resistors and the blue wire to one of the other lugs. (BTW, you made the same drawing error on the 68K grid resistor that connects to V1-pin 2.)
You always have 1.5K connected, so the switch in the OFF position will give 1.5K total NFB resistance. Selecting the 1.8K puts it in parallel with the 1.5K which yields 818Ω. Selecting the two 1K puts 500Ω in parallel with the 1.5K and this yields 375Ω.
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I completely misread that.
You are referring to the lug, not the switch....
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(BTW, you made the same drawing error on the 68K grid resistor that connects to V1-pin 2.)
I was not planning on grounding that one. I am going to use one of these non-grounded lugs.
BTW I've updated the layout, using your input.
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BTW it is going to be an amp head.
If everything works out right it should look something like this (knobs not being flat of course).
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The fact that the 3-position switch for the NFB would not have the middle value in the middle, kept nagging me.
So I have changed it to a version with an On-Z-On switch, so now the regular Blackface value is the middle one.
It will lead to the following values:
-450 Ohms
-820 Ohms
-1,64 K Ohms
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Sounds interesting. I've never heard of an On-Z-On switch. Could you post some switch details, or a link to a data sheet, or link to where you bought it?
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Sounds interesting. I've never heard of an On-Z-On switch. Could you post some switch details, or a link to a data sheet, or link to where you bought it?
Sure:
This is where I found it (sorry, everything in Dutch):
https://www.newtone-online.nl/catalog/product_info.php?cPath=22_24&products_id=1908 (https://www.newtone-online.nl/catalog/product_info.php?cPath=22_24&products_id=1908)
But the only info there is the image I added here.
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Due to the holidays none of the parts will arrive before mid January. So I started working on the cabinet.
It will be a 1x12" that I will combine with a rotary speaker cabinet (an old speaker from an early eighties Yamaha organ. Same as the ones used in the old Yamaha RA-100).
I have a spare Celestion A-type lying around. If that doesn't sound nice I'll have to look for something else (suggestions always welcome).
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Your cab looks very nice. One tip: your T-nuts are mounted on the inside of the baffle board. This orientation will force you to bolt the speaker from the outside. Which will work IF you do not have speaker cloth installed and if you will never ever remove the speaker. I suggest flipping four of the T-nuts around to the outside (that's all you really need to mount the speaker anyway), so that you can bolt the speaker from inside the cabinet.
You can paint the outside of the baffle board black, along with the T-nuts, and no-one will see the hardware though your grill cloth.
For your future builds, these special mounting screws are worth every penny: https://www.tubesandmore.com/products/screw-8-32-12-reverse-thread-speaker-baffle-mount (https://www.tubesandmore.com/products/screw-8-32-12-reverse-thread-speaker-baffle-mount)
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Ah... I know. The baffle hasn't been connected yet. I just put it in to see how hard it is the get in and out with the TV-front and back-braces.
I am also planning on painting both the baffle and the inside of the cab black.
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Looks like a cool build.
I'd be very interested to learn some more about your choice of amp name.
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Thanks. If all goes well, the final stack should look like shown in this image.
Top to bottom:
-Amp head
-1x12" cabinet
-Rotary cabinet
The name doesn't have much of a meaning. The project started with the idea of combining a Blackface pre-amp with a tweed power-amp (AB763 pre + 5F6 power). A Fender "Bastard", if you like. Since "Tweed Bastard" sounds too much like a spoiled English Lord, I went with "Black Bastard".
But, as it is now the amp will just be a regular AB763, with a few minor tweaks.
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Just back from a family trip to Berlin (yes, we played Bowie and U2) and ordered all the parts for the amp.
Finished the intended 1x12 cab, and started on the headshell.
Put a Mojotone Anthem 12" in it. Damn, that speaker sounds great with my Supro DualTone build. Even better than the Celestion A-type that currently is loaded in that amp.
The overly complex backpanel of the cab allows me to add an additional cab in parallel or series. The volume knob is connected to a 100 Watt L-pad (the switch bypasses it). That way I can reduce the volume if I want, because most 12" speakers are more efficient than the Yamaha rotary speakers that I used (on the photograph the bottom cabinet).
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At last. Most of the parts arrived, so I can start.
First thing I did: hammering in the turrets on the circuit board.
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Finally had time to work on the amp (sometimes life gets in the way).
Most of the soldering has been done:
-The circuit board with all the leads at the bottom
-All the wiring to the front and back panel
Btw when soldering all the resistors and capacitors I made sure that all resistors are oriented the same way (easier for reading the values) and all capacitors have their values pointing up (again, easier for reading in the future).
Still to be done:
-Wire-up the transformers
-Change the circuit board spacers, because the current ones are too high
-Wire-up the circuit board to the rest of the amp
-Finish the headshell (but for that I have to wait for the weather to clear up, because I have to use the router outside).
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I updated the layout in the first post, because the wires were connected to the wrong legs of the master volume.
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And I updated the layout drawings again, because I made a stupid mistake: the polarity of the two big power capacitors are the wrong way around.
I found out the hard way, when I fired up the amp.
Had nice blue fireworks in the rectifier and a very smelly "pop" from the capacitors. So, now I need to get new ones.
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Magic smoke = magic learning . . .
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Well, at least it provided me with a learning/Zen moment.
Won't be able to get new capacitors before Wednesday, so I had some time to work on the rest.
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Well, I replaced the capacitors, but I got another problem in return.
When I start the amp up, after a few seconds there is a very loud crackling sound (continuously, but without any apparent pattern).
After a approximately 30 seconds I get a single blue flash in the rectifier tube (this used to say "output transformer", which was a mistake) and the mains fuse of the amp is blown.
I've tried to track down the problem, but I cannot seem to find cause of it. All the wiring seems to be OK.
Any guess what the cause could be:
-Faulty rectifier tube?
-Blown capacitor (that knows how to hide it)?
-Shouldn't have connected that Orange cable of the 290CEX OT to ground?
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I get a single blue flash in the output transformer
:huh: Never seen that before. Do you mean like an arc or spark and it's really on the OT? Exactly where do you see this blue flash? Try to describe it better.
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Sorry, I meant a single blue flash in the rectifier tube.
The output transformer seems to be OK (really don't know why I type OT instead of rectifier).
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Most likely suspects are the tube and/or filter caps.
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Thanks...
I'll have to see when I can replace them.
Best desolder them first and measure them.
OK, so I'll have to replace all the 22uF/50V Elco's in my amp (I replaced the two 70uF/400V today).
BTW
In your schematic you are using 32uF caps. Rob is using 22uF caps. What would be the effect of that value difference?
Don't have a spare GZ34 lying around.
Would it be safe to use a 5Y3, just for testing puposes?
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Just desoldered the filter caps; they all have the correct value (so I soldered them back).
Also tried a 5y3 for a few seconds, but had the same noise.
So, I am back to peering at wires and drawings.
Is there a way to measure whether the OT is still Ok?
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Blowing fuses here is not a good way to go about things.
Until everything has proved good, it’s a really good idea to use light bulb limiter to protect the amp from fault current.
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Well, at least I found the culprit that caused the blowing fuses: I forgot the ground wire that runs from 47uF, 100V cap in the bias circuit.
No more blowing fuses and nice blue light.
Now I have to solve another problem, because there is a loud hum coming from the amp, which is louder than the guitar.
The loudness of the hum depends on the volume and master knobs.
If I turn the master past halfway I get a whistling sound as well.
So, the quest for finding my f#$@k ups continues.
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Found my f^$#^k up.
I didn't ground the input jack properly. In the image on the left you can see what it should be (and what I drew), on the right what I made. Really don't know what I was thinking when I did that :violent1:
Tonight I will see whether repairing that will solve all the humming and buzzing (fingers crossed).
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Problems solved...
Next to the mistake with the input wiring I also omitted the 47Ohm NFB tail resistor to ground.
Glorious Fendery sounds (still need to bias it properly, though).
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I just noticed that there is a weird, buzzy distortion when I turn the preamp volume up. If I turn it up completely it almost becomes a whistle.
As if some part is rattling inside causing it to buzz.
If I bypass the EQ completely it is mostly gone (and you get a huge volume increase) and you get a nice overdrive.
As it is now, the amp is more than loud enough for my purposes, but it shouldn't do that.
Any idea what could cause that?
Not the power section, because I get it also when the master volume is turned down and I turn up the preamp volume.
Its also not any of the tubes, because when I changed these for different ones the buzz persisted.
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Finally all problems solved (with regards to this amp)!
By looking a bit more carefully at the schematic (and the wiring inside the amp) I was able to track down what caused the buzzing: a faulty cap.
By forgetting the 47 Ohm resistor to ground, the 100 nF capacitor got a little more than it could handle. It still worked, but could not handle higher output from the preamp anymore.
If you look closely, it developed a faint red line.
I didn't have a 100 nF cap at hand, but three 33 nF ones in parallel do the job just fine.
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And the amp is finally done.
Today I had time to tolex the headshell and put everything together.
I'm really happy with the results. It sounds great as well.
Everybody thanks for the help and feedback.
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The NFB should be connected directly to a tap on the secondary, not after the selector switch. So, you would still use 47Ω for 8Ω tap or 100Ω for 4Ω tap. This way the NFB will remain constant regardless of which tap the speaker load is connected to.
In that same line of thought you should use 27Ω when connected top the 16Ω tab?
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The problems with the build kept bugging me. Furthermore, having a lot of connections to the bottom of the board would make any future maintenance next to impossible.
So I decided to redesign the board and completely replace it (and most of the wiring). No more wires running underneath the board. More logical grouping of the amps functional blocks (Bias board - power section - phase invert - preamp). Layout optimized for having short leads.
And, of course, no more connections at the underside of the board.
The amp is now quieter as well.
Still sounding fantastic.