Help convert the Rocket to bass amp

[sethyrish]

I’ve started playing bass through my rocket and quite like the sound apart from occasional squealing and other noise. Not sure what's causing it. (Harmonics??) I don't mind any sag, distortion or overdrive.
Is it possible to make it more bass friendly?
I’ve already built a Greenboy 15/6/1 bass cab and don’t mind changing caps, resistors even pots. I’d prefer to keep transformers and tubes.
Power transformer
Magnetic components. inc
Classic Tone 40-18065
300v/260v
5Volt 5A
6.3v7.5A
Output Transformer
Triode Electronics
A-470-S 35 Watts
Valves
Rectifier GZ34
Power EL-84 x 4
Preamp 12AX7 x 3

It's quite expensive building stuff in Australia as we don't seem to make any components anymore, so it usually requires $300 just for postage from the US or Europe every time I want to make anything, hence my desire to convert. Plus it's just for jamming with friends in my lounge room.

Any ideas?

[Stevem]

So if you play guitar thru it no squealing, but only with Bass?

At best you have 15 watts of clean power to mess with and for me a Bass amp even just for home jamming with a Drummer I would want a 50 watt amp, but hey , to each his own!

[rp]

Don't see many 4XEL84 bass amps, maybe for a reason, but why not try it. Maybe those fat bottle EL84 (JJ?) that can take 400V? Maybe try ss rectifier, use the 260V tap, use an 80mf for C1 if you didn't already, I wouldn't go too high with the filtering, you might want fat tone more than dry punch, like the old portaflexs. Not grounding the bass pot (IIRC the Vox has it grounded? visa versa?) ie getting rid of the interaction in the tone stack. Maybe playing with the 10K mid resistor.

First thing though is to get rid of the squealing.

[sethyrish]

Thanks for responses.
Yes, only the bass seems to induce intermittent squeal but there is a very fast quiet ticking sound with both guitars. I guess maybe one of the valves is faulty. I'll do some valve swapping tomorrow.
The OT, A-470-S can handle 70watts. Would swapping the 4XEL84 for two KT88 work or would that mean changing everything?

[rp]

There are posts here about all sorts of Rocket output stage variants including iirc KT88. You need to make sure you have enough heater current for KT88s. The drive might be ok for KT-88s, your have to change some output stage resistors, certainly the bias. I wouldn't chew it all up, make a good guitar amp into an iffy bass amp, no going back. Is it for home, recording, coffee house?

I'd do those few reversible things I mentioned and maybe add fixed bias too, tap it off the OT secondary, look at some Marshalls and the BF Princeton. Put it on a switch with cathode and fixed, bias a little cool for bass.

Traynor made something like it, EL84 fixed bias, Bassmate I think.

Sounds pretty interesting actually, way easy to switch back and forth, guitar rocket or bass rocket.

-some sort of high voltage EL84Ms (TAD STRs? Google around)
-Mojotone SS rectifier plug-in thing
-fixed bias, maybe fixed/cathode bias switch
-unground the bass pot, or use a switche pot, there's Vox info on this on the forums - google Vox bass pot mistake etc.
-Add a temp 25K mid pot for the 10K and tweak to taste for bass then replace with a fixed resistor, or put that on a switch too, use a guitar dpdt pot.

[Phil_S]

It's quite expensive building stuff in Australia as we don't seem to make any components anymore, so it usually requires $300 just for postage from the US or Europe every time I want to make anything...

Any ideas?

At that price, I'd think you can find a winder who will make you something really terrific and for less than $300.

What about Pacific rim suppliers? There's got to be someone in China or SE Asia who has something you can use! Heck, try eBay!

[M Fowler]

Good job on the Rocket bass amp. I suspect cold solder joints is the problem. there is no NFB so that isn't the squealing problem.

I built a Rocket bass amp too but used octal power section and SS recto, gave the amp to my daughter.

[sethyrish]

Thanks for all the info.
That should keep me busy researching for a while.