Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Guitar-Tube-1969 on June 18, 2016, 08:22:02 am
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Hi all,
I'm from Germany and new to this great forum. I've built several pedals and one small 3W tube amp from kits by now.
My next project should be an AB763 Deluxe w/o trem/rev. I've found Sluckeys schematic (AB763_Deluxe_Lite.pdf) in this forum which will be perfect except:
- I need an european version with 240 V
- I want a version with more clean headroom => 6L6
Do you can help me out with an appropriate PT/OT (preferable 4, 8 and 16 ohm)?
In addition I've got the question what parts to change in your circuit in addition to different tubes, PT and OT?
Thanks - Axel
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I believe the fender Pro, is similar in design to the deluxe, it has 6l6 tubes in place of the 6v6s. The schematics also provide output bias info.
Regarding o/ts take the fender part numbers, and go to the Hammond website, it will provide the specs for the transformers for both the deluxe and the pro. Hammond will also suggest upgrades.
While I haven't provided the specific details, I hope this information leads you in the direction you want to go.
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Pick out the 6L6 AB763 amp you want then go to the Hammond website and find the replacement PT, OT, and choke. There will be choices for 240V (Euro) PTs as well. Write down the model numbers you want then go to your favorite dealer. You should be able to find several dealers in Europe.
Here's the Hammond link...
https://www.hammfg.com/electronics/transformers/classic (https://www.hammfg.com/electronics/transformers/classic)
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Thanks for your input.
I've found a PT
http://www.tubeampdoctor.com/de/shop_Netztrafos_Netztrafos_fuer_Fender/Netztrafo_fuer_Fender_Deluxe_Reverb_Grand_Stack_DEL_MAXIMO_4840 (http://www.tubeampdoctor.com/de/shop_Netztrafos_Netztrafos_fuer_Fender/Netztrafo_fuer_Fender_Deluxe_Reverb_Grand_Stack_DEL_MAXIMO_4840)
with the following specs:
Primary: #72500
0V White
230V Black
240V Brown
Secondary:
350V/160mA Red
56V Blue
0V Green/Yellow
350V Red
3.15V/4,8A Green
0V Green/Yellow
3.15V Green
5V/3A Yellow
5V Yellow
In addition I want to use the following OT:
http://www.tubeampdoctor.com/de/shop_Ausgangsuebertrager_Ausgangsue_fuer_Fender/Ausgangsuebertrager_2x_6L6GC_5881_fuer_2_4_und_8_Ohm_153 (http://www.tubeampdoctor.com/de/shop_Ausgangsuebertrager_Ausgangsue_fuer_Fender/Ausgangsuebertrager_2x_6L6GC_5881_fuer_2_4_und_8_Ohm_153)
which seems to be a replacement for the 45W super reverb circuit.
Two question now:
1. Will this combination of PT/OT fit to Sluckeys AB763 Lite circuit?
2. Do I have to change other components in that circuit?
Greetings from Germany - Axel
Thanks in advance
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Secondary:350V/160mA Red <---this seems oversized for a Deluxe. OK, you want to run 2 * 6L6. Still pretty large, this seems like a transformer for a Twin Reverb = 4 * 6L6. Check physical size if this is what you want.
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That PT will give you more B+ voltage than you will probably want. Can't you find those Hammonds in Europe?
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I found the following Hammonds on tubetown:
290 DEX (super reverb):
http://www.tube-town.net/ttstore/Transformatoren/Netztrafos/Fender/Hammond-290DEX-Fender-Power-125P5D-Super-Reverb::2151.html (http://www.tube-town.net/ttstore/Transformatoren/Netztrafos/Fender/Hammond-290DEX-Fender-Power-125P5D-Super-Reverb::2151.html)
290 CEX (vibrolux):
http://www.tube-town.net/ttstore/Transformatoren/Netztrafos/Fender/Hammond-290CEX-Fender-Power-125P26A-Vibrolux::2150.html (http://www.tube-town.net/ttstore/Transformatoren/Netztrafos/Fender/Hammond-290CEX-Fender-Power-125P26A-Vibrolux::2150.html)
Will be these ones a better choice and which one will fit better for a AB763 with much clean headroom?
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I would use the 290 DEX.
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Thanks!
Is there anyone having a partslist für Sluckeys AB763 lite?
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I bet you can make a parts list in less than 15 minutes.
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Ok, will it fit in a 5e3 or Princeton chassis?
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Ok, will it fit in a 5e3 or Princeton chassis?
Depends on dimensions of the transformers. I could build the Deluxe Lite in a blank 12" x 6-1/2" x 2-1/2" chassis.
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Ok, I decided to build your original version with the 6V6 first to learn more about the circuit. After understanding more from it, I will alter it to a version with more output. For that reason I will use the following transformers/choke:
PT: Hammond-290BEX
OT: Hammond-1760H
Choke: Hammond-194A
When preparing my parts list I detect some difference between your schematic and the layout diagram where I'm not quite sure:
- The filter caps C12-C16 are not marked with a "+" in the schematic, but I think these of course have to be electrolytic capacitors, right?
- C17, C18 are 50uF in the schematic and 47uF in the layout diagram, I would youse 47uF/100V right?
- C1, C7 are not marked with a "+" in the schematic, but I think this have to be electrolytic capacitors too, right?
Do you have any suggestions regarding the capacitors (manufacturer, oil cap, sprague Orange Drop, mustard, ...)?
Thanks in advance - Axel
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Right to all electrolytic questions.
Those yellow caps on my layout are Mallory 150s. They are scaled properly. I know that Xicon chicklet caps will fit nicely. I know that Sprague ODs will not fit nicely. I don't know about any other type caps.
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great, thanks!!
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Do you have any suggestions about the cables?
I would prefer to use a shielded cable as shown in the layout diagram.
For all other connection I will use 18 gauge material which I think is 1mm diameter.
Is that OK or is it better to use different gauges for e.g. the filaments...?
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I like RG-316 for shielded cable. RG-174 is OK also but it's not as durable as 316. I use 20AWG for filaments ang 22AWg for everything else.
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Sluckey, are values for C10 and C11 0.1 or 0.047
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Sluckey, are values for C10 and C11 0.1 or 0.047
Ha ha! They are .1µFs on the schematic (like Fender) but .047µFs on the layout (my preference).
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So, I've got my list, added the resistor and capacitor numbers in sluckeys layout diagram and got four new questions:
Question 1:
R27 and R28 are not shown on the layout diagram, I suppose
- R27 goes from V4/8 to ground 2
- R28 goes from V3/8 to ground 2
I added this in the attached layout diagram, is that right?
Question 2:
V3/4 and V4/4 both goes to B
I added this in the attached layout diagram, is that right?
Question 3:
I would use 5W resistors for R29, R30, R32, R33, R34 and 2W for all other resistors.
Is that OK or do you have different recommendations?
Question 4:
The position of the standby-switch in sluckeys version is different from that from the hoffman single channel version (see attached diagram). Why is this so and what effect does this have?
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OK, a little background on my drawings... None of the 'layout' drawings in any of my documents are a complete layout drawing. They are meant to be used to build the boards and show how the boards interconnect with the other components. You can print (full size) any of my board layouts, tape it to a blank board and use it as a drill guide to make an exact copy of the drawing. If you only had one of my board layout drawings to work with, you probably could not build a successful amp. YOU MUST REFER TO THE SCHEMATIC FOR THE COMPLETE AMP WIRING.
About this particular drawing... It is old and today I would have provided clearer instructions for grounding. Here's how I'd do the grounding on that amp. Screw a ground lug to the chassis very near the input jacks. This is the preamp ground. Put a ground lug on one of the PT bolts. This is the power ground. Screw a ground lug to the chassis near the entry point of the mains cord (or IEC). This is the safety ground. Connect the green wire from the power cord (or IEC) to the safety ground. DON'T CONNECT ANYTHING ELSE TO THE SAFETY GROUND. Now referring to page 2 and 3 of my pdf, connect gnd 1 and gnd 3 to the preamp ground lug. Connect gnd 2, A gnd, B gnd, and the output tube cathode resistors to the power ground.
And your specific questions...
1. Connect R27 and R28 to power ground.
2. That's wrong. Connect both pins 6 together and run a single wire to B cap.
3. 1 watt is fine for R29, R30, R32, R33, and R34, but I would use 3 watt metal oxide. 1/2 watt metal film is fine for everything else. Use Mallory 150s or Xicon coupling caps. Other types may not fit the board very well.
4. My STBY switch is wired just like the original. Either way will work.
Sounds like you're about ready to roll. Keep us posted. We're interested in your project. BTW, I have never actually built this Lite version, but several members have. It will easily fit a 12 x 6-1/2 x 2-1/2 chassis.
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Great, thanks! You see, I'm a beginner.
just 2 questions more:
1. I can't get 3 watt metal oxyd resistors for R29, R30, R32, R33, and R34. Only 2 Watt or 5 Watt. What would you recommend?
2. I did not found metal film resistors (0.5 watt) for R27, R28 (1 Ohm) and R15 (100 Ohm). Can I use metal oxyd 1watt instead?
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Sluckey, why the two 20 uf 500volt caps in parallel for node A. I know that Fender shows about the same but doesn't that give you 10 uf for the initial cap. for the rectifier. I think either is rated for 60uf input.
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For Guitar-Tube-1969... (Your questions don't seem to be beginner questions.)
1. I would use 2 watt.
2. The 1Ω resistors need to be 1% tolerance. Wattage doesn't really matter. These resistors are only used to help you set the bias. They are not required for the amp to operate correctly. R15 is only 1/2 watt. Use whatever you can find. Note... R15 should be 47Ω if you plan to connect NFB loop to an 8Ω speaker tap (like the Deluxe Reverb). It should be 100Ω if connected to a 2Ω or 4Ω speaker tap. Ain't no big deal if you get it wrong.
For labb...
Two 20µF caps in parallel will give you 40µF total capacitance, not 10µF.
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Whoops, got it backwards don't I.
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Hey Sluckey,
I've got a question for soldering the PT to the rectifier tube.
My PT is the Hammond 290 BEX which has
2 green cables (6,3 V) => filaments V1-V4
2 yellow cables (5V) => filament for rectifier (pin 2 and 8)
till here it is clear, but...
2 red cables (330V/660V) => for rectifier pin 6 and 4 ??
1 red/yellow cable => to power ground?
thanks in advance - Axel
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2 red cables (330V/660V) => for rectifier pin 6 and 4 ?? yes
1 red/yellow cable => to power ground? yes
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So, I finished the amp except an impedance switch for the speaker out (hopefully I will get the package soon). I will shorten the cables from the OT after assembling the switch. I'd added some pictures from the result.
Now I will double check all of my soldering against the circuit and then I will put in the tubes and power on.
Do you have some hints for starting / measuring at first?
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Build 1 of these light bulb limiters and use it on 1st power up to test for shorts;
http://sluckeyamps.com/misc/Amp_Scrapbook.pdf (http://sluckeyamps.com/misc/Amp_Scrapbook.pdf)
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Hey Sluckey, that was a good tip!
Unfortunately i've got a short as it seems.
Power on => OK, bulb shines very low
stdby on after a while => NOK! bulb shines very bright
stdby on shortly after power on => OK at first, NOK! (same result as before) after a short while.
I've double checked everything but I can't find the defect.
Do you have an advice, where to look for?
I've attached the final fotos.
Thanks in advance - Axel
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Keep the amp plugged into the lamp limiter for now. Pull the output tubes. Measure voltage on pins 3, 4, and 5. What have you?
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V3-3 / ground: 399 VDC
V3-4 / ground: 397 VDC
V3-5 / ground: -29,7 VDC
V3-3 / ground: 399 VDC
V3-4 / ground: 397 VDC
V3-5 / ground: -29,7 VDC
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How did the lamp limiter look?
Those voltages look good. For now just set the bias pot for max negative voltage on pin 5. Put the 6V6s back in and try again.
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Now after switch on the standby the lamps is a little bit lighter than before. I can plugin and hear the guitar but also a lot of hum. This is with and without the guitar plug in.
Max neg. voltage on pin 5 is -51.1
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Is the lamp anywhere close to being full brightness? By that I mean as bright as it would be if it were plugged into a standard lamp.
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No, not at all.
It seems that there is a problem with the high input jack. I only hear a guitar signal from the low input. This signal is week an a little bit distorted
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You should be done with the lamp limiter. Plug the amp straight into the wall and turn it on. Watch for smoke, listen for odd sounds. And smell for odd odors. Be ready to turn it off if any alarms.
If all is well, set the bias a little hotter. Adjust the pot for about 25mV on pin 8 of each 6V6. Then measure the voltage on pin 3 of each 6V6. What have you?
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Ok, nothing smokes, but....
Increasing the bias leads to much hum from the speaker. I can only turn it up to around 10.1mV on V3/8 and 8.6mV on V4/8. Why are these different?
V3/3 = V4/4 = 432 VDC
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The voltages are different because the tubes are not perfectly matched. Nothing to worry about. But the hum is an issue. Hum can be caused by many issues, bad tubes, filter caps, NFB, poor connections/solder joints, etc. For now, disconnect the NFB wire that runs from the speaker jack to the board and leave it disconnected. Does the hum go away?
Pull the preamp tube. If the amp still hums, the problem is in the power amp and/or power supply. If the hum goes away, the problem is in the preamp stages.
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Ok, the hum goes by pulling V1. Nothing changes when disconnecting the cable from the speaker out tip. So it is a problem in the preamp stage?
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Could it be a grounding problem of the pots and the input jack? I change the soldering from two input jacks to one to test if it comes from a wrong soldering there. But I get no input signal now.
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Yes, sounds like it's a preamp problem. For now just leave V1 out. Reconnect the NFB wire and try to set the bias for about 25mV on pin 8 of the output tubes. Then measure voltage on pin 3 of the output tubes. What do you have?
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It feeps again when biasing up. I can reach 12.0 mV on V3 and 10.0mV on V4. Pin 3 then have again 431 VDC
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What does 'feeps' mean? Does it quit feeping if you disconnect the green NFB wire at the board?
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If I disconnect the nub wire, there is no feeping and I can adjust the bias at 25mV
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Hey all,
I have started to rewire the filaments in a better way and when I finished it I will recheck the preamp section.
Now I will be on a vacation till August 14th. After my return I will go on and post my results.
Thanks for your advice!!
Axel
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If I disconnect the nub wire, there is no feeping and I can adjust the bias at 25mV
OK. The permanent cure for that is to swap the OT primary wires on pins 3 of the 6V6s. Then reconnect the NFB wire. Now readjust the bias pot for 25mV on pins 8. Measure the voltage on pins 3. What are the voltages.
Now plug in the preamp tube and tackle the hum problem if it still exists.
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Hey Sluckey and all others,
I'm back from vacation and continue building...
It seems that I have solve the power amp/supply:
After changing the OT primary wires as Sluckey stated I could bias to nearly 25mV and receive the following values:
V3/8: 25,5mV
V3/3: 410 VDC
V3/4: 413 VDC
V3/5: -40,1 VDC
V3/8: 25,0mV
V3/3: 410 VDC
V3/4: 412 VDC
V3/5: -40,1 VDC
There is nearly no hum, even at high volumes, but... there is no audio signal. If I plug in a guitar cable and touch the tip, there is no signal at all.
I have checked the circuit against my soldering twice, but I cannot find the problem.
Do you have any idea how I can proceed?
Thanks - Axel
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I have checked the circuit against my soldering twice, but I cannot find the problem.
Check it again and again until you find the error. There is very likely a wiring error or wrong component value. Click the link in my signature line below for some good info on verifying your schematic agrees with your layout and agrees with your actual amp.
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Hey Sluckey,
finally I got it.
The shielded cable that goes from the input jack to V1 was broken inside. After replacing it, it seems to work fine. Tomorrow I will check the rest, bias and will add a bright switch ...
I will upload the final result and hopefully a sound sample.
Thank you so much Sluckey for your advice.
Axel
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Do you have any advices for biasing this amp? I',m a little bit confused from the information I read...
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Read this...
http://www.duncanamps.com/technical/lvbias.html (http://www.duncanamps.com/technical/lvbias.html)
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Ok, thanks for the link!
I tried it, the amp was on for about 2-3 hours now. The bias values are:
1st measuring:
V3/3: 411 VDC
V3/8: 25,0mV
=> 411*0,025=10,3W
V4/3: 411 VDC
V4/8: 24,0mV
=> 411*0,024 = 9,9W
2nd measuring in between:
V3/3: 408 VDC
V3/8: 25,0mV
=> 408*0,025=10,2W
V4/3: 408 VDC
V4/8: 23,05mV
=> 408*0,0235 = 9,7W
3rd measuring at the end (same as the first):
V3/3: 411 VDC
V3/8: 25,0mV
=> 411*0,025=10,3W
V4/3: 411 VDC
V4/8: 24,0mV
=> 411*0,024 = 9,9W
Because 6V6 has a max. rating at 12W (hot), it seems ok, isn't it?
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2nd question: I want to add an extension speaker jack.
The wiring of the main speaker jack at the moment is:
G: wired to the black cable from the OT and to the ground
S: wired to G
T: Green wire from R24 (820 Ohm) and green wire from OT (in my example the three wires for 4, 8, 16 Ohm goes to an impedance switch and one green cable from there to the tip)
Could I use another jack and wire it as follows?
G: wired to G of the main speaker jack
S: not connected
T: wired to the tip of the main speaker jack
Then they shoud be parallel wired and I could:
- Two 8 Ohm speakers, impedance switch at 4 Ohm
- Two 16 Ohm speakers, switch at 8 Ohm
Is that right?
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Because 6V6 has a max. rating at 12W (hot), it seems ok, isn't it?
The info I have says a 6V6 PAMAX is 14 watts.
Your thoughts about adding a second speaker jack are correct. Just remember, if you only plug into one jack it has to be the main jack.
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Great, thanks!
Would you recommend to bias them a bit hotter, e.g. 12 Watt?
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You are biased at 70% now. That's what I have my fixed biased amps set for. But you can bias hotter if you like the sound. If so, you may have to change tubes more often.
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Hey all,
I've got a problem with my power tubes. Both tubes show a silver film at the inner glass (see attachement). The tubes were new and only have about 20h playing time. What's that?
In addition the internal speaker lets bell my rectifier tube on a certain frequency. It disappears when using an external speaker, so it seems to be a physical problem, any ideas?
Thanks - Axel
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(Nearly) ALL tubes have that silver splash. Yours did from new, right?
It is "getter". A monoatomic film of thirsty metals to absorb the last traces of gas inside the vacuum.
Related question:
In time (decades) the getter can turn brown. As long as there is ANY silver left, it is still doing its job. Throwing-out a tube just because its getter is 10% brown around the edges is like throwing out a soap-bar when 10% is gone (and 90% of soap is left).
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Great, thanks!
Do you have any idea, to prevent the tubes (either rectifier or power) to resonate at a certain frequency?
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Here is a final foto with faceplate and cab - Thanks to all of you!!
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It's possible that the particular tube just resonates (rings) at a certain frequency. Since it only does it with the internal speaker I would try another tube first. This is called sympathetic vibration. I've seen silicone rubber rings for small tubes that help. Maybe the same rings are available for larger tubes. Can you stop the ringing by holding the tube with an oven mitt or pot holder?
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That amp turned out very nice! :thumbsup:
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Hi Sluckey,
sorry for my delayed answer but we play a short gig and I used the amp. Sounds good!
To the resonance problem: The ringing stops when I'm holding the tube. It stops too when using an external cab only. So it seems to be a physical problem, isn't it?
Axel
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and: thanks for your compliment!!
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Hey all!
I've uploaded a video on youtube with a clean sound example (first solo, at 1:40 with a backing track). The video contains some photos from the final assembly.
Have fun - Axel
https://www.youtube.com/watch?v=j9rUS5LV6r4 (https://www.youtube.com/watch?v=j9rUS5LV6r4)
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Nice playing! Did those rings on the octal tubes solve the 'ringing' problem?
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Thanks.
To be honest, I'm not sure. I've changed the rectifier and power tubes to JJs and did not check it without the rings. The is no ringing at all the moment and I'm happy.