FI SONIC 30WGR RESTORATION

[jazbo8]

Read the last two paragraphs here. To balance them, you would also need to use two separate cathode resistors/capacitors, not sure if it is worth the trouble since they will drift off balance as the tubes age anyway. Some imbalance contributes a bit more distortion which it is not necessary a bad thing in a guitar amp (within reason of course).

[TIMBO]

Thanks jazbo, I was thinking of doing that as I do it to all of my other builds. I do have a match pair of 6L6s but was curious of the effect that a mismatch would have. Thanks

[TIMBO]

Hi guys, Transformer has arrived and its a great fit to the old bell ends. The newy is a laydown type with all the wires on one side.New specs are 325-0-325@180mAs,5v@3A and 6.3v@4.5A which are similar to the original.

[TIMBO]

Hi guys, transformer is in and fired up  Checked voltages and all look good. With nothing plugged in just a faint hum even when volumes are turned up.
When the reverb is turned OFF (not via foot sw) the voltages at each node is increased by 15v the readings are :-
REVERB OFF
NODE A - 380v
         B - 353v
         C - 294v
         D - 255v
         E - 239v
These are about what they should be with a 5U4GB rect
With these voltages I calculate the power output to be about 32w
Please check my calcs..
Plate - 372v
Screen - 351v
Cathode - 28.9v
Across 1ohm res. 52.6mA
Between Plate and cathode - 348v
I'm just a bit concerned that the OT is rated at 25w and as this is built to a minimum standard that it might do damage.

So far so good 

[TIMBO]

The so far so good has gotten even better  . This amp sounds fantastic. Not a hint of BUZZ or HUM. Had a small issue with one of the volume pots being a bit scratchy and intermittent so I pulled it apart for a clean and bobs your uncle, just still a little scratchy now. Also the input jacks had corrosion inside so they were replaced and alls good. I also changed two of the grid resistors on the inputs to 33k, just for the effect.

I was concerned that it may be putting out too many watts for the OT having calculated 18w per tube and having a 25w OT but after playing awhile the OT was warm but some of that was coming from the 6L6s

As I have posted the reverb tank I think is dead and not worth trying to fix as tanks are not very expensive and a replacement is a 4FB2A1C with an input of 1475ohms and an output of 2250ohms, vertical mount as there is no room for a horizontal tank.

My only real concern is the tube sockets are very loose (octals were OK)and after pushing the grips( the things inside that grip onto the tube pins)together with the point of a probe they were a little better, I even tried to spread the tube pins a bit and this also helped but not as good as I had hoped. The option of replacing the old sockets is not one that I would look forward too, any quick fixes for this.

As said, to me the amp sounds VINTAGE with a very articulate crisp sound with nice sustain and when cranked can do a rock chord with out any problems.
The two channels have there own sound one brite and crispy and the other a little dark but with a warm feel (this one suits the blues,YUMMY)
I think the speaker cab suits the head very well, no bass buzzes or rattles is also a little dark but can hit the highs, also YUMMY
It always seems that any of these old amps that have vibrato have there problems and don't work, I was lucky with this one and did not have to touch a thing.

To me the amp is working fine and when the CC resistor have drifted as much as some have maybe this has added some of that VINTAGE MOJO  and at this stage I won't be changing any to the original value, unless anyone can see a particular resistor that may need to be replaced for safety and avoiding any damage to the circuit Thanks

 

[sluckey]

As I have posted the reverb tank I think is dead and not worth trying to fix as tanks are not very expensive and a replacement is a 4FB2A1C with an input of 1475ohms and an output of 2250ohms, vertical mount as there is no room for a horizontal tank.

Since your tank is transformer driven I'm betting the input is 8Ω. See if you can get your hands on a 4AB3C1B typical Fender tank to patch in before you buy that high impedance 4FB2A1C.

You can check the input requirements by checking the dc resistance of the secondary of the driver transformer with the tank unplugged. If it is under 10Ω DCR then you don't need a high impedance input tank.

[TIMBO]

Thanks again Sluckey, Just measured is 70.2ohm

[TIMBO]

Hi guys, The new tank has arrived and is now fitted. Fitting it between the transom and the bead at the bottom gives better clearance between the tank and 6BM8 tube.

I like reverb a lot this has plenty, @1 just a slight hint and @10 it goes a little nuts, so @5 is a good amount so at this stage I don't think it needs to be tweaked.

Gave the cab some TLC and is looking great 

Tubes......
All preamp and PI are mullards and are sounding great.
Power tubes JJ6L6GC also sounding great.

DID someone mention KT66s, been hearing a lot about these and they look PRETTY cool but can't find much about cathode biased tubes.

What are a good value for money tubes and can they be cathode biased.Thanks

[jazbo8]

Since the 6BM8 has a triode and a pentode in the same envelope, you can't just plug in any old pentode/tetrode in there. So for the same pinout, you can use ECL82 or 6F3P (Chinese or Russian).

[TIMBO]

Hi guys, Got some new tubes in the mail, a pair of Tung Sol 6L6G and a GZ34. A new set of voltages taken and all looks great.

A small concern with the power sw., it sparks when turning from standby to ON. I'm sure in time this may burn the contacts and possible replacement of the sw. Is there any thing I can do to reduce this to avoid having to replace it.

DO I replace the old grille cloth on the head cab to match the speaker cab???????

Thanks to all for a great project.

[TIMBO]

Some final pics.......

[TIMBO]

Some more...............

[DummyLoad]

very nice work! 

 

--pete

[TIMBO]

Hi guys, Had a chance to crank it up a bit and its not good :sad2:The volume pot on channel 1 is scratchy (worn) and this causes crackling and if not turned down to 0 when playing channel 2 also causes crackling. Channel 2 volume not too bad just a small spot that crackles, so a couple of new pots needed.

When playing notes on G,B and E its great, lots of tone and clarity and when turned up only just a hint of breakup and some sustain (this is good) but more would be better

But when playing notes on E,A and D lots of flabbiness and wont play power chords.

With this problem the vibration causes the valves to rattle as they are gripped by the retainers but are loose in the socket. I had feared that the sockets may have to be replaced as I can't be sure that the tension on the valve pins won't loosen in time.

I do not have another amp that I can compare this amp with or any original recordings so can anyone give me a comparison going by the schem to another type of amp.

I think I need to address these couple of problems. Thanks

 

[TIMBO]

Hi guys , got some time to tidy up a few unfinished jobs. I needed to replace the two volume pots as they were very scratchy and was causing a few problems.

This has made the amp come alive tone wise as well as volume.
With the old pots, I would have to get it to about 5 before I got any real volume, now by the time it hit 2 its more than loud enough for a bedroom. 

I also increased the cathode resistor on the power tubes to see if this might reduce the volume a bit but at 2 its still loud, I will leave it for the moment as it will also reduce the output and not over work the OT.

I still have a problem with the tube sockets being loose, so i'll have to think about it some more as I don't want to try and replace the sockets. Thanks everyone,its a real gem 

[Willabe]

I needed to replace the two volume pots as they were very scratchy and was causing a few problems. This has made the amp come alive tone wise as well as volume.

That's interesting, not only volume but tone improvement, good catch.

its a real gem 

Alright then, you worked it all out and your efforts paid off and then some.


             Brad     

[TIMBO]

Thanks Brad, yeah the next thing is the sockets. I'm not able to get the same as what been used. so I'm at a loss at what to do. The only other option is to custom a retainer 

[TIMBO]

Loose tubes sorted. This was what I thought was the best solution and its worked well.

I'm trying some 6V6s as the 6l6s are way too loud for my small room and I was hoping the get a bit earlier breakup at a lower volume.
I got a pair of GTs with 427v on the plates, 31v on the cathode, 400v plate to cathode pumping out about 26w. I think that this is a little too high for them so I might try a bigger cathode resistor. I got a pair of Tung Sols i'll try. This project is just about done. 

[Willabe]

That looks good, nice and neat.


            Brad