Roland Bolt-60 reconditioning

[Bolt60]

Hi,

i started here but decided to open a thread for having specific discussion about repairs and mods i made (or should make, please advise) to the Bolt-60 a friend entrusted me.

the standby switch position is corrected.

Old tubes are still making it but ... very tired (low volume power...) 5881/6L6WGC (instead of OEM 6L6GC) and a RCA 12AT7.



replacement tubes will be matched JJ 6L6GC pair (could you please tell me signification about sticker right side readings data ?)
and a balanced JJ 12AT7 (ECC81) for phase inverter.



Very bad job was done to this amp, most to the PCB but wait ... look at these 2 MKT capacitors added in a "beautiful technical specialist artwork..."



should have been made like this !
at same time and for the very low cost involved i soldered in a IC socket and replaced the CD4016 with CD 4066 (better specifications)




The "should be" 100Kohms next to the phase inverter tube was reading in ... Mohms so i replaced all 1/2W carbon compound resistor with 2W carbon film.
Maybe i should replace also the 600V capacitors and little tantalum blue one... but testing say they are ok so let's go as is.






Recap job of power supply board, all 1/2W carbon compound resistor replaced with 2W carbon film.
6,8K bias CC resistor replaced with 4,7K 2W CF in serie with 5K cermet 1,7W potentiometer (for external bias adjustment, i will add external banana 2mm for 1 ohm voltage test points)



Here are the 1ohm resistors between pin 8 off each tube and ground.



picture attachment number limit...

[Bolt60]

Reconditioning of the very tired Effect LOOP switch, the OEM job is not as "art rules" requirements
switch case is not soldered to the PCB so contact solder joint take all the mecahnical stress when switch is used
desoldering job



Time to open it carrefully


Switch porn



oxydation everywhere and spring contacts are too soft,
spring are reconditionned by using tiny pliers very carrefully !


and oxydation problem is solved with this product, a toothbrush and elbowgrease...



Time to resolder it ... as it should have been done, case soldered to pcb !

[Bolt60]

I deplaced the main arrival to 240V transformer coil (instead of 220V)
now i have normal voltage (before this i had 510V instead of 490V at CT transformer for exemple)

[Bolt60]

Ok time for some questions

Bias voltage can now be adjusted from -50 to -45V
at this last value, 1 ohm voltage show old tubes current unbalance (one is 38mA other 29mA)
I think CT transformer should be part of the problem as his coils are unbalanced (85ohms and 75ohms...)

first question:

can i tweak 220K R11/R12 tubes resistor to re-balance ? or i'm wrong ?
It would be a pity with matched JJ 6L6GC to have their good balancing killed by this CT transformer coil unbalance

Now the current is adjustable from 25mA to 38mA, with 478V plate this gives 12 to 18W range.
Even with the tired tubes sound is more pleasant (to my non musician ears) and now the volume is WAY louder with same potentiometer position !
position 3 is already very loud (but sound still ok)

I read than for a 6L6GC with 25W max dissipation (maximum life), 480V plate, AB class should go (50%) from 26mA to Hot 47mA (90%) within warm at 36mA (70%)
Sound seems good to me at 38mA but it's the max on my bias adjustement and still far from 47mA...
I will lower 4,7K value sérialed to the bias pot adjustment for having more range on the Hot side (if needed sometime),
25 to 45mA range would be great.

May i ask specialists what you think about it please ?
any help advise appreciated.

[Jazz P Bass]

Swap the tubes in the sockets to see if it is the tube or the circuit.

Personally, I would not worry about the 'imbalance'.

As a matter of fact, play the amp & then switch the tubes from socket to socket & see if it sounds different.

[Bolt60]

thanks, should have thinked about it

result > problem is tube imbalance which is a very good news as it should be solved with matched ones.

now i have to try lowering 4,7K bias resistor to 3,3K or maybe 2,2K and if range is ok then will install/tweak new tubes.

still have the overdrive to repair...

[Enzo]

The two halves of the output transformer have different resistances, 85 and 75 ohms. First, that is not much of a difference, and second, even larger differences are often normal, due to the way the thing is wound. You are measuring the wire resistance inside it, and if some windings are wound atop others, their wires will be longer for the same number of turns, and so the resistance higher. This is not a defect.



25w 6L6GC? Seems to me they have been 30w tubes for a long time.

I agree a 240v transformer is probably preferable, but the difference between 490v and 510v is small, less than 5%. Remember too that the high voltage is not regulated. At 240v mains, your B+ is a little more than twice the amount, so for every volt the mains changes, your B+ will change two. A 5v mains change is a 10v B+ change. So your B+ can change hour by hour. Over here there is a four times difference, so count your blessings.

Just remember this is only a guitar amp, not a precision lab instrument. Small imbalances are not critical.

[Bolt60]

yes it's not class A HIFI amplifier though i like state of the art jobs and to the most i'm learning so ... questions, questions and more questions

I'm aware 25W is not 6L6GC maximum, as said it's my choice for tube longevity (and tone) as this amp is going to play a lot.
thanks for your answers and advise.

[Bolt60]

Bias adjust (33mA to 50mA) with 4,7K +5K pot



Average before reconditionning

PIN3: 478V
PIN5:-39V
Av1:25mA
Av2:19mA

Average after reconditionning and new tubes:

PIN3: 481V
PIN5:-50V
Av1:33mA
Av2:33mA


I noticed volume is very loud and there is imbalance growing (Av1:60mA and Av2:30mA instant. at VOL 4) the more i go upper level 2
on volume potentiometer.
I suppose loud volume is because i'm using a smartphone player signal output (which is already amplified) and not a guitar, i'm right ?
growing imbalance ? socket pin springness contacts ?
thanks,

[ilcaccillo]

Bias adjust (33mA to 50mA) with 4,7K +5K pot
I suppose loud volume is because i'm using a smartphone player signal output (which is already amplified) and not a guitar, i'm right ?
growing imbalance ? socket pin springness contacts ?
thanks,

Old thread but I'm at the Roland Bolt subject at the moment.

To connect a smartphone that outputs umbalanced consumer Line Level, you should connect it to the Low input that is better suited for line level sources like keyboards and drum machines.

Do you still have your amp?