Fender Pro Amp clone

For some years I have been thinking to make me a Fender style amp head. Peter Green and Danny Kirwan have been great influence on the Fender tones I have learned to admire. My earliest interest in Fender amps was actually Ampeg J-20 that really was a nice version of Fender Deluxe 6G3. I liked the idea of 6V6 power tubes but I found the tone controls a bit too limiting, just a single Tone control. But I knew the amp had to be Brownface style (generally lower negative feedback than in Blackface amps). Then recently I stumbled upon a few Youtube videos that showed Brownface Fenders and really liked the Pro Amp tones. It has no reverb but I don't really care about spring reverb in amps. But this amp has a spectacular tremolo effect almost like a univibe.

Fender Pro Amp (6G5-A) was in production in 1960-1963. It was a combo amp with a 15" speaker. Some amps had tube rectifier and some solid state rectifier. My amp will be a head version of solid state rectified one.

So I started to figure out the list of parts I need. All info I had about the parts is in the layout and schematic available in the internet. I found out I could use a Blackface Deluxe reverb chassis (wrong color but I can live with that) and head cabinet as they are easily available. The Reverb knob would just be changed to Presence knob.

IMPORTANT NOTE!

The original layout diagram of Fender Pro amp 6G5-A found in the internet has a serious error. The wire from +448V connection point to the left side 5881 power tube should go to pin 6, not pin 5. If you are building this amp, make sure you solder that wire to pin 6.

Here's the discussion

https://el34world.com/Forum/index.php?topic=19073.0

And here's the corrected layout

http://el34world.com/charts/Schematics/files/fender/Fender_Pro_6G5A_Layout.pdf

I mistakenly ordered PCB power amp sockets but that's okay as I can modify them easily to non-PCB style.

24.11.2019

I ordered some more stuff. Fuse holders, power tube retainers, the 7025 preamp tubes and 25uF/100V cap for the bias board. The written value in the layout and schematic 25uF/50V is surprisingly low voltage as the voltage across the cap exceeds 55 V. I guess the the 50V cap could take it but I am not taking any chances as the the cost is low.

I still don't have the transformers and the choke (and head cabinet). I will order them later.

17.12.2019

Ok. It's been sometime since last update. I haven't been doing much since I was waiting for the rest of the potentimeters to arrive. I didn't find them from Europe so I had to order them from US (two 350k audio pots with 70k tap, a 3M  audio pot, a 10M audio pot ordered from Amprepairparts.com ). They arrived yesterday.

It was quite interesting how my package made the journey. It took 7 days to cross the Atlantic ocean , then waited 5 days in Denmark and finally waited 12 days in Finland before delivery. I am not blaming anyone, just saying. The pots arrived safe and without any damage.

Above: I installed the can multi capacitor. I found quite convenient can cap and thought it would help me to get rid of the "dog house" that you usually find in Fender amps. The hole for the rectifier tube was almost perfect for the can cap. Just enlarged it a bit to make safe distance between the chassis and the can cap terminals. I still have to add 2 larger caps with the can cap. Also installed fuse holders for the HT and mains and a bias pot.

Above: All pots and input jacks installed. Also some wiring started.

20.12.2019

Almost finished the green heater wiring. I made the artificial center tap with two 100 ohm resistors although I later noticed that my power transformer will have a real center tap for the heaters. I'll finish the heater wiring (pilot light is still not connected) when I get the power transformer installed.

21.12.2019

Today I made great progress. I soldered the needed caps and resistors to the pots and finished almost all tube socket wiring. Also attached two new larger caps to the chassis. They will be kept in place under the bias/rectifier board.

22.12.2019

Today I started to figure out how to make the power supply without making the dog house. All the 6G5-A layout picture is telling is the color codes of the wires going to dog house. I started studying the schematics to find out what color of wire corresponds to each node in the power supply. I hope got it right.

As I mentioned earlier I use a can cap and then two additional single caps: 40uF+20uF+20uF+20uF and then 22uF and 22uF.

I also added 1500 ohm grid stopper resistors as they are the modern way to do these things. And a 68nF/630V cap across the standby terminals to I guess prevent the pop when turned on.

Ceriatone 6G6-B layout picture has been great help.

http://www.ceriatone.com/ceriatone/wp-content/uploads/2016/12/6G6B_22Dec2016.jpg

Then finally I ordered the Classic Tone transformers from Amppartsdirect.com. A power transformer with multiple primary options. To be exact, we have 230 V wall outlet in Finland. So I am gonna use the 240 V primary winding.  Then I ordered the output transformer with multiple secondary options 4/8/16 ohms as I need the 4 or the 16 ohms usually. The model numbers: power transformer 40-18029, output transformer 40-18006. 

I am not expecting them to arrive very soon but I hope they will be here during January. More than anything, I hope the transformers will arrive safe and undamaged.

Then I also ordered choke and the head cabinet + a few small parts from the local amp parts shop Uraltone. The head cabinet is Deluxe reverb head cabinet as the chassis is also Deluxe reverb chassis.

1.1.2020

Today I soldered some more wiring. This time on the pot side of the of the amp. I think is good idea to wait the transformers at this point. Before soldering the wires from pots and tube sockets to the board. My transformers are approaching. Today they left Copenhagen. It would be great to have them here next week.

Also, on Monday I fetched the head cabinet and the choke. The cabinet sure looks and feels great.

12.1.2020

My transformers arrived yesterday and I fetched them from our local post office. The transformers were carefully packed and the package was unharmed. Thank you Amppartsdirect.com !

So today I was very eager to start attaching the transformers and the choke. It all went without any big difficulties. I had to enlarge the opening for the power transformer as mine was a bit larger that the normal Deluxe Reverb power transformer. As you remember, this is a Deluxe Reverb chassis. I went on to solder all the transformer and choke wires. It became apparent, the can cap came very close the power transformer. The clamp touches the transformer so there is 1 mm gap between the cap and transformer. I hope it won't cause any issues. It will warm the can cap somewhat but maybe not enough to cause premature failure. The second power tube is there too to warm up things more or less.

I was thinking it would be good idea to first try the lower HT voltage available on the power transformer. There are 305 V and 355 V taps available.

I also attached the power cable as I made all other power wiring too. I added HT fuse as it's again the modern and safer way to do these things. Also added 220k ohm bleeder resistor on the first 40 uF cap. Just to be safe.

Above: Transformers and choke attached to the chassis. I tried to use the holes already drilled as much as possible.

Above: Output transformer secondaries soldered to the output jacks. 4, 8 and 16 ohm outputs.

Above: Lead dress? Wiring doesn't look very tidy. Partly because I haven't cut some of the wires to the length yet. While I'm aware that certain wires are good to keep away from others, it's enough for me if the amp works as it's supposed and is relatively hum free. 

Just now I realized I have to add the pedal jack for the "vibrato" (it's not really a vibrato, it's a tremolo or maybe a tremolo-phaser in Pro Amp's case) .

18.1.2020

I attached the main board and I finished the wiring during a couple of days. I also noticed I still need to order some parts. For example I broke the mains fuse holder so that it cannot be attached to the chassis anymore.

After studying how to switch on the power for the first time, I took a chance and connected the amp to my Current Limiter and to power outlet. This should be done without any tubes.

Rob Robinette's great info page

https://robrobinette.com/Tube_Amp_Startup.htm

I was happy to see I had no short in the power supply. I measured all voltages where they should be present.

Above: first power on without tubes.

Above: Then added preamp tubes and finally power tubes.

After attaching power tubes, connecting a load (attenuator+speaker cabinet) and measuring the voltages, I took one step further and connected a guitar to the input.  (At this point it is good to remove the Current Limiter and connect the amp straight to wall outlet. This is because the Current limiter will lower the voltages below normal working conditions and make the amp sound very saggy and crappy.) 

Ok, switched the amp on. The sound was very distorted and weak. Switched the amp off and started to figure out why. I checked the tube socket connections and found that I didn't solder two pins on V5 and V6. Pins 3 and 8 should be tied together on both sockets and I failed to solder the pin 8 connections. Ok, fixed that and the sound was much better but still not normal. I also noticed voltages were pretty low for this amp and thought that I should switch to higher voltage power transformer secondary. The one I was using (on purpose) was 305V-0-305V. The higher voltage is 355V-0-355V. At the same time I was a bit worried that my filter caps were 500V. Then I calculated that the voltage should not go over 500V. As matter fact the voltage went to around 490V when Standby on and then Standby off (playing position) went to around 440V which was about the normal for this amp. But now the amp was squealing (oscillating) and I knew I had to swap the output transformer primary wires. And I did and it helped.

After awhile I noticed that the other power tube was red plating (the plate of the tube is so hot that its starts to turn red) and should fix that next. I need to increase the negative bias voltage on the power tubes.

Later same day: I found out the other tube socket didn't get negative bias voltage so it's bias ran away and tube started red plating. I soldered the connection again and replaced the grid stopper resistor just to make sure it would be working right. After this the red plating stopped and I could bias the power tubes properly. The plate voltage arrived at 458V so I set the bias pot around 40mA of plate current. The amp sounds quite good. There is still something going on with the vibrato channel treble pot. It doesn't behave like the normal channel treble pot does.

Above: The inside view of the chassis

Above: Tubes and transformers etc.

Above: Only the right color pilot light jewel (I wanted amber) and model sticker is missing

Last update for today: I found a loose treble cap in the vibrato channel. Soldered it properly and the vibrato channel got the same bite as the normal channel.

I must say that I am very happy to been able to solve every issue I have faced. Needles to say I have again learned a lot about building amps. The final task is to attach the chassis to the head cabinet.

19.1.2020

I measured some important voltages in my amp. Here's the link to the document.

https://docs.google.com/spreadsheets/d/13cB9GM9qOkPxVd4Hr12dPdPkQ-WxJdAAn6_gohjrWM4/edit?usp=sharing

Audio demo

https://youtu.be/Rdt9mDTP72Y

25.1.2020

Yesterday I receiver the chassis straps from Tube Amp Doctor so I could attach the chassis to the head cabinet. I made a template of the chassis so I could drill the holes to the cabinet. It all went smoothly. I also attached some aluminum tape to the cabinet  to increase shielding against radio interference.

I added some cosmetics, Pro amp model name. Just taped it to the chassis. It looks a bit rough but I don't mind. It's better than the empty space there. The Presence control tag is wrong color of course but I will leave it until I get something better.

5.2.2020

I have been playing the amp for awhile now and started to feel the amp could benefit from a fan moving air around the power tubes. The chassis is getting quite hot and that is because the hot air from the power tubes goes up to the chassis. This is rarely a problem in the amps that have power tubes above the chassis.

So I bought a cheap 12 V fan and mounted it below the power tubes. The fan is powered from 6,3 VAC filament voltage by a half wave rectifier (a diode) and a 470uF/16V capacitor. The fan gets about 8,5 V of power so it runs a bit slower and also more quiet. Here's the idea explained except that I have the fan mounted different place. The fan is mounted to the bottom of the head cabinet.

http://www.blueglow.de/FenFan.html

I also put a jack and plug to the power cable so I can disconnect the fan from chassis and to turn the fan off when needed. 

30.3.2020

I have a fx loop in almost all of my amps (except the Divided by 13 RSA23 and I don't want to mod it in anyway) so I was thinking to have fx loop in this one too. So I ordered Metropoulos Amplification Zero Loss fx loop kit. I have installed two of these loops before to Marshall amps and as classic Fenders are not that much different, I was thinking I would be able to do it. I studied the amp schematics and the installation instructions, it should not be that difficult. Let's see when I get it. It should be here in a week or two.

25.4.2020

No sign of my Zero Loss fx loop. It's been stuck somewhere since 4th of April. I can only hope it will move on someday. In the meantime I ordered two Proco cables from Parts Express. They left on 14th of April and arrived yesterday but it was a different courier.

29.4.2020

My Zero Loss fx loop moved yesterday. Though it's still in US. Good to know it's not lost.

4.6.2020

The package hasn't moved after 29th of April. International consumer package shipping seems to be completely halted in US because of the Covic-19 pandemia. I just hope it will arrive someday. Not going to hold my breath.

24.6.2020

Surprise! My Zero Loss fx loop is in the Finnish customs. I cleared it today and I'm waiting for it to arrive tomorrow or day after tomorrow. Though I'm not going to try to install it before my summer holiday begins, after two weeks.

26.6.2020

Ok. I fetched my Zero Loss fx loop today from a local Post Office almost 3 months after it was shipped. I will continue the project after my summer holiday starts.

13.7.2020

The first day of my summer vacation. I noticed the other day, when I plugged my Roland VK-7 into my Pro Amp without the attenuator, the background hum was quite substantial. Earlier in the spring time I noticed that I had not payed attention to the orientation of my coupling/signal capacitors. I learned that these non-polarized capacitors have a property called 'outer foil'. And that you should orientate the cap so that the outer foil is to the low impedance of the circuit. Aiken amplifications has great article on the subject.

www.aikenamps.com/index.php/where-to-connect-the-outside-foil-on-capacitors

So I noticed that I had quite a few of my caps wrong way. Today I switched them around. But there are still a few cap positions that I am not sure how to orientate them. They are the ones that had originally ceramic caps but I had to substitute them with polyester caps that have outer foils. As far as I know ceramic caps don't have outer foils as they are not made out of foil.

So I switched around the ones that were wrong way and listened if it made some difference. Well the background noise is still quite pervasive but I feel it could be a bit lower than before. I always play this amp full up with attenuator so the noise is not bothering me too much. If I ever need to play it louder, I should get the noise down a fair bit.

9.3.2025

I'm happy to say I have progressed solving the issues I had in the amp that I finished 5 years ago. A year ago already, I did replace the cramped main board with a bit larger one. The dimensions of the new board are 30cm x 8cm. The new board allowed the components to be installed without too much effort. Of course I had to buy all new resistors and capacitors but you live and you learn.

The bigger issue was the enormous hum I had in the amp because apparently I had no idea how to make the grounding in the amp. It's still not totally hum free but it has a lot less of it. So much less that if I'm using an attenuator and cranking the amp full up, the quite normal white noise is completely masking the hum. The fact that the hum does not increase with the volume knob, tells that the hum is not inducing in the preamp. 

What caused me to grab my soldering iron again and start solving the hum issue was Psionic Audio's recent Youtube videos about ground schemes.

Ground Schemes | Pt. 2: Vintage Fenders

https://www.youtube.com/watch?v=v0GMtPbvNgw

Ground Schemes | Pt. 3: Bus wires in Fenders

https://www.youtube.com/watch?v=lRizkiviA9k

I carefully watched the videos and familiarized myself the points Psionic Audio was making. I'm not trying to make an all-inclusive list of the important points here. Please watch the videos.

Btw, I never installed the Zero Loss fx loop. It's not something I definitely need in this amp. I put it in my Marshall instead. 

The amp sounds wonderful and now with a lot less hum.