Changing rectifier in a Ceriatone 5f6A Bassman

[pickerg]

Hi all,

I'm new to this and just trying to pick up knowlege that I read so please bare with me. I own a Ceriatone Bassman head that was professionally made. I really like the sound but I think it's not quite there with how I'd like it to sound and its very powerful. It has 12AX7 preamp tubes, GZ34 rectifier and 6L6s. After reading round I think i'd like to try 5881 power tubes and possibly change the rectifier to a 5V4, 5R4 or 5U4 with a possible change to 12AY7 in V1.

Does anyone know if the 5U4 would cause problems for the transformer from the Ceriatone kit because of the current it draws? Would I be better with one of the other options? Anyone done anything similar?

Also would the above changes (along with a re-bias from an amp tech) lower the overall output at all? The reason I ask this is that I would like to try with a Celestion Blue in a 2x12 so would be limited to around 30 watts.

Thanks.

[tedmich]

greetings! What exactly would you like to achieve with these mods? Reading between the lines the amp is too loud and doesn't quite sound "correct" to you, if this right? The 5881 are a lower gain tube and will mellow the output a bit (but not much!) they should breakup faster than the 6L6s though. The 5U4 should sub in but I'd ask the Mfg. directly if the PT can handle the added current. Are you hoping to get more dirt with the 5U4? Setting the bias correctly is also a very good idea, as it can have a major impact on the amps tone.

[The Dude]

Just thinking out loud. If you're trying to substantially decrease the overall volume of the amp and the plate voltage is low enough, you might consider modding it for 6V6's.

[pickerg]

Thanks for the replies. I know how the amp sounds is subjective but the best way I can describe is that I think it could sound sweeter if that makes sense, to me it sounds slightly harsh. While doing this if I could lower the overall output and sweet spot where the amp is just breaking up that would be a bonus, I'm not looking for all out overdrive.

[pdf64]

A 5R4 has similar sag / voltage drop as a 5U4 and draws 2 amps heater current.
Older info lists them as having an 8uF reservoir limit.
I understand that was revised upward on later info (from Enzo) and there is much anecdotal evidence that they sub in fine for regular guitar amps.
However, they are quite large tubes, so check that they will fit in the cab.

Have you got the right load impedance connected to the amp? The stock 2 ohm is unusual.

You might consider reducing the degree of global negative feedback on the power amplifier, as that can result in the onset of distortion being harsher than otherwise. This might be achieved by increasing the value of the 27k feedback resistor, to eg 47k or 100k.
Be aware that will increase the gain of the power amp, which will be perceived as making the amp louder for a given volume setting; however, the power output will not increase, so it will overdrive at the same SPL, though a lower number on the volume control.
Pete

[pickerg]

Ok, so thanks to a helpful email from Ceriatone, I now know that the 5v tap is only rated for 2A so that rules out trying a 5U4 unless I changed PT. The amp has bias points so I could probably adjust the bias myself, but I'd really need to know the plate voltage with the current GZ34 rectifier. Looking online people seem to have had good results with a 5V4 that would give a smaller voltage drop than a 5U4. Once I know the plate voltages then I can see if a 5881 is suitable. I am thinking along the correct lines here? (the PT gives off 340V at the taps)

Thanks for the suggestion of 6V6 tubes, but it might take significant mods to get the plate voltage low enough?

Yes I'm more looking to mellow the output a little, I realise the perceived volume will not be much different between the different power tubes but if I could lower where the sweet spot of just breaking up comes (actual spl not just on the dial) that would be what I'm hoping for.

If it makes no difference or doesn't give me the sound I'm looking for then at least I'm learning lots on the way!!

Thanks for taking the time to read and for your comments.

[ron vogel]

5V4 is generally 20-25 volts higher output than a 5U4, 5Y3 would be closer...but 5R4 would probably be better. I've tried them all in my Princeton clone. You may want to check out tweaking the phase inverter, or seeing if the preamp voltages are way high too. Might be a simpler fix for the tone you are trying to get to.

[bluto]

FWIW there is a handy chart showing the specs for various rectifier tubes at Rectifier Tube Voltage Drop Chart | 300guitars.com
As was said be careful swapping rectifier tubes that bump up the B+, I learned the hard way and fried an old 6L6G...fortunately I realized what was happening and shut the amp off before it burnt out the OT

[pdf64]

My finding is that this chart https://www.kcanostubes.com/content/...eability-chart is more realistic for our purpose.
Pete