[Blues Custom Mods]

On 4/27/2014 at 7:38 PM, Andyroid said:

I've just found another design flaw that may be the sole reason the BC30 self destructs. There is not enough limiting resistance before the rectifier. Limiting resistance helps keep ripple current within limits and is stated on the datasheet.

 

 

It is worth noting that the choke will slightly lower the required limiting resistance, but not by much. The above datasheet chart provides the calculation necessary to determine the limiting resistance provided by the transformer:

 

Rs = Rsec + N2Rpri

 

I have measured Rpri to be 5.5R and Rsec to be 37.5R (75R for the entire winding) so...

 

Transformer ratio N = 350 / 240 = 1.45 so...

 

37.5 + (1.45 * 1.45 * 5.5) = 49R

 

The chart shows that for a 350V tap, the limiting resistance needs to be around 105R for EACH PLATE of the rectifier, and this is for a fresh valve manufactured to 1959 standards. So lets assume 115R will be safer for modern 5AR4s,

 

115R - 49R = 66R

 

68R is the nearest standard resistor, two of these should be chassis mounted inside the amp, and the transformer secondary taps (350V) should be wired directly to these and then the other ends of the resistors wired to the PCB. I recommend 25W types, as the voltage rating should be in the region of 550V. The working voltage in practice will be much much lower than this but at the moment of power on, it will be higher and it's nice to know that nothing can go wrong! I over spec almost everything in my amps!

 

BTW, using the standby switch makes the problem worse because the rectifier is fully ready to conduct. If you are unlucky enough to flip the switch at the moment the mains AC voltage is at it's peak then destruction is almost guaranteed. Starting the amp from cold (without the standby) will slightly reduce the likelihood of failure. So, for a happy BC30, don't use the standby and install limiting resistors!

 

I did some measurements by putting an ampere meter between de sockets of one of the fuses. I measured 63mA in triode mode. Playing guitar and driving a signal through the valves didn't influence the current.

Between the 2 coils I measured 670V RMS

If I'm not mistaken, the total dissipated power would be the square of 0.063A * 68 Ohm which is 0,27 Watt. I bought 2 resistors of 25W so I guess they'll do the job.

I hope that the drop of 4.29V caused by each resistor doesn't make a big difference for the characteristics of the valves.

Has someone tried this mod and what is the potential risk if I put those resistors in line? 

[Blues Custom Mods]

I tried out the new mods in the band setup. C3, C5 and C6 together with C15 kills the dynamics totally

I play a guitar with P-90s 

The overall sound was overcompressed loosing bottom and high ends. 

It was like playing with a compressor on steroids. 

Neil Young's music requires control of the dynamics. 

I'm planning to put C3 and C6 caps back and definitely revert to the C15 22uF since the bottom end of my sound spectrum went on holiday

I'm just wondering which of the 3 brightness caps I best place back. I mainly play on the drive channel, so I guess most impact is expected from C5 and 6. Is that a correct assumption? Shall I start to put back C6?

Problem is I cannot test this at the fullest in my living room since dynamics of the amp change when level is cranked up. So an indication in advance might help to pick the right cap to put back

R39 replacement is next on the list and shortcutting the standby switch 

[Blues Custom Mods]

3 hours ago, Matttjuk said:

Another update as this thread has seems to have died 🤣

R21 is now 1.1k and C15 2.2uf, made a nice difference to the drive channel and the tone controls seem a lot more responsive.

So list of mods I've done are:

Master volume  (R39)

R15 now 1.5k

R21 now 1.1k

C15 now 2.2uf

C3, C5 and C6 removed

Standby switch deleted

Fan power moved from valve supply area to before the 5 diode's (V1 - V5)

That'll do I reckon, very happy how it sounds now.

 

 I'm in the middle of the mods. 
First round, I did the most rewarded combination of C3,C5,C6,R15,R21,C15 (according to my synthesis of this forum thread)
The amp has less highs now, but EQ is more controllable.
I need to try it out during the rehearsals tomorrow where it is cranked up and input signal boosted by my GT-5 pedal board.
I was planning to do the R39 mod, but I don't know which channels it impacts. If this allows to crank up the clean channel and controlling the master volume with it (potmeter), than this might produce an even rounder overdrive.
I'm working in the direction of the Fender 5E3 sound (which never will succeed completely). But this amp fits in my budget to use it for lead guitar in a Neil Young tribute band.

A side project will be the control of the pot meters with servo motors. My Boss GT-5 is a versatile and robust beast that has some nice MIDI features. Those midi messages will be taken in by an Arduino and this will control servo motors which will drive the potmeters.

This will allow me to tweak the sound from soft and harder rhythm parts to  a "full howling drive with delay" lead sound for solos.
I can use 5 patches and the express pedal to program whatever I want 😉

I'll just keep this posted here. It always can add more context to the splendid information already available here. 

BTW I always play in triode mode.

[Blues Custom Mods]

Hi,

I'm new here, but read the complete thread thoroughly several times. I had the idea to summarize all suggestions and put them on github in a asciidoc project. People can contribute and modify the already available content. It would be nice if I could have a high resolution PDF from the modded scheme including the transformer output resistor mod.

In what application is the schema made in and is it available in it's original format?

I have the most important mods summarized in a text file, which I will convert into asciidoc so that it can be generated in a nice PDF.

Pictures of the mods made are welcome, they can be included in the project

I will put the project in github.

I have my BC30 for more than 5 years now. The first replacement was with Groove tubes (don't recall the exact type since labels fell off due to heat)

Now they were replaced by JJ Electronic's and the amp sounds harsh again and breaks up much much later in the drive channel.

I use the amp only for Neil Young tribute in the lead part, so I need a warm creamy overdrive that's gently saturated. The groove tubes did the job much much better. I play mainly on a Hagstrom Super Swede P90 tremar and as backup with a Yamaha AES-620HB with Seymour-Duncan HB in the bridge position. I put the amp in class A mode.

I also had a generic question about removing components. Is it possible to de-solder them without taking out the PCB completely?