Inspiration needed for Octal Bassman...

[norris2002]

... or maybe a deluxe... or maybe something else all together.

I am new to this forum, but have been hanging around for a little while soaking up info on the builds going on, and looking into the trainwreck stuff also. I have built a champ recently, and plenty of effects pedals and quite a few guitars in the past. I enjoy building and would like to try something a little bigger. I would like something that I can play both guitar and bass through, and will happily take a few pedals up front, and the obvious choice for me seems to be a Bassman. The reason that I want Octals is that I like the idea of an all octal amp, and was going to work from the ever popular 5f6-a schematic, but replace the 12a-7's with 6sl7's. My questions are these:

Are there any circuit tweaks needed to do this successfully, or is it a strait tube swap?

The 1st stage of both channels of the 5f6a have the cathodes tied together, would splitting these and having one cathode (guitar channel) with typical 1500R / 25uf values, and the other channel (bass?) with higher cap or res values? My aim here is to have the two channels as dedicated guitar and bass, rather than bright and normal.

Will the 6sl7 work in the PI as well as it would in the gain stages?

What other octals will I be able to substitute? (6sc7 has tied cathodes, and I do not know too many other octal triodes used in guitar amps).

I hope to get some useful comments.

Thanks
Neil.

[norris2002]

and I was going to use a solid state rectifier as I feel this will help with the bass side of the amp. Perhaps I would be better off here still using the GZ34, but implementing Gerald Weber's Diode bypass on the bottom of the socket.

Neil.

[M Fowler]

https://tubeamparchive.com/viewtopic.ph ... al+bassman

[David Root]

That's the way to do ane early tweed all right. You will notice Mark Fowker say he was "surprised how quiet it was".

This amp is only 18W. You mentioned a 5F6A, which is 40W. You can do a 5F6A with 6SL7s like you are thinking, however, it might be noisier than the 5B6 Mark built. If that happens, consider a 6V DC supply for the 6SL7 heaters.

I have built higher gain amps than the 5F6A with 6SL7s, even an ODS, and there 6V DC heaters are essential as this tube, like all octals, were not designed for modern gain levels and also don't have the humbucking 12A*7 heater wiring either.

I've not done the 5F6A with 6SL7s so I can't say for sure if you would need DC heaters, just my $0.02 based on some experience with 6SL7s.

[KT66]

Neil, welcome to the amp garage. The 5F6-A / plexi circuit is well known as a good guitar amp, but as a bass amp it leaves a lot to be desired. I'll bet Mark's 5B6 is a nice blues amp - cathode biased, paraphase PI, octal preamp and all, but probably flubbier than the 5F6-A for bass. Interestingly enough, this amp: http://schematicheaven.com/ampegamps/b1 ... taflex.pdf uses 6SL7s, a paraphase PI, and a tube recto but was/is a popular bass amp. Apart from the different circuit, 2 reasons for this are the closed back cab and the Baxandal tone stack. Check out the links in this thread to see the bass amp that Bob-I built with a bax : https://tubeamparchive.com/viewtopic.php?t=9644.

My point to all of this is that if you try to make a valve amp that does both bass and guitar well you probably aren't going to get the best of both worlds. But if you want to try anyway, I would make the bass channel with a bax. You could even do the 5F6-A and add a 3rd channel like the B15 pre that you could toggle in with a switch.

Here is an amp that some bass players have told me they liked that is derivative of the 5F6-A : http://schematicheaven.com/newamps/sunn_model_t.pdf Basically the same except for the tone stack values and the big clean 6550s in UL mode.

I also agree with David that you might have a noise issue and should consider DC heaters.

[ampgeek]

Greetings Neil!

I did exactly that with a 5F6A for my very first amp build. All octal pre including the PI and 5881s in the PA. I did use 5691s which is a more industrially "hardened" 6SL7 so I have no specific experience with the latter unfortunately.

The only significant changes to the standard 5F6A plan were as follows:

Split the V1 cathode and used 10 (normal) and 5 (bright) uf bypass caps
Hardwired V1 control grids (pins 2 and 7) at the tube socket
Eliminated the "death cap"

The hardwired grids on V1 might not meet your guitar/bass desires. But..it is VERY useful for guitar applications to get the right brightness when jumping back and forth between single coil and HB equiped guitars.

Other than that (and re-wiring changes to match the 5691 pin-out) it is pretty much dead stock 59 Bassman in a 2X12 combo cabinet.

It is, to this day, my favorite amp. Very full and round tone and as quiet as a mouse. It also takes pedals excellently. Thankfully on that that as it is a prettly low gain affair and needs to get to ear splitting volumes to get any kind of "dirt" without them!

Never plugged in a bass guitar but will give that a shot and let you know what I learn.

Cheers,
Dave O.

[Andy Le Blanc]

Octal tubes should be fine, you'll have to watch the heater to cathode rating
on a 6sl7 as a cathode follower, but thats pretty simple with a hum pot.
elevate the heaters 50 to 75 v dc and adjust the hum balance.
And the hum will be reduced without having to go dc.
It might be a good idea to use a 6sn7 as the inverter, less gain but better tone.
You could also try a 6sj7 pre, or try running both sides of a 6sl7.
The amp will break up a little later than with 12 ax7 but clean isn't a dirty word.

[norris2002]

Thanks for all the responses. Other forums IU have been on have not been this helpful, and so quickly.

I was planning to build this into a head so I can pair it with different speaker cabs depending on the situation. I would like the amp to work with my bass, but I am not a bass player, and would not gig with a bass and this amp. It is more so that I can noodle around with my bass downstairs and get a half acceptable sound out of it.

The comments on DC heaters / raising the heaters make sense to me, and I think I will build the amp as per the schematic, with raised heaters, and tweak with different tone stacks etc. until I am happy.

Where is a good source for parts (mainly iron) in the UK? I have used Bluebell before, but wondered what everyone else tended to use? I dont want to pay for shipping from the US (and import duties), and this doesnt make sense with the 120/240v difference in the PT.

Neil.

[Richie]

the early tweed fenders used the 6SC7, it has a shared cathode. And bit less gain. And seems about any of the octals, you can go through a few and find good ones. Some will be crap, and make all kinds of noise.

So go through some to find good ones. The octals can turn out a great amp though, the tone is very good. The 6sn7 or 6SL7, again go through some, because you'll see or find some will be much better than others.

[ampgeek]

My lucky-day, NOS vacuum tube "acquisition" (free!) of ~ 2500 tubes included about a dozen NOS RCA red base 5691s. Every one of them sounds great in all position in my octal Bassman.

They are cited as an industrially hardened version of the 6SL7 with, IIRC, 20,000 hr longevity. I see them going for $20 to $40 each on ebay these days.

Depending on how many gigs/year one plays, it might make economic sense focus on this species from the very beginning.

Just thinking out loud!

Dave O.

[norris2002]

As with most things I build, I will plan lots, and change my mind lots, and have all sorts of ideas before going back to what I originally intended to do. I have some other projects that I need to finish before starting on this, and will need to convince the bank manager (read: girlfriend) to release the funds, but when I do start on this, I will make sure I document the process on this forum, and most likely ask lots of questions along the way.

Thanks to everyone for their tips and tricks, and if anyone finds a good source of bassman iron in the UK, let me know

I am currently trying to decide on the colour combo that I am going to build my amps with. Current thinking is black tolex / vinyl with basketweave grill cloth, or cream with oxblood?

I digress...

[Andy Le Blanc]

With a spray application of adhesive, you can use any cotton fabric you want,
any color, any print, get your fiscal manager into it and she can color coordinate.

[Cliff Schecht]

This thread gets me excited about my upcoming 807 Bassman build. I'm thinking 5F6A for the first one, because my first tube amp (59 Bassman RI) was stolen (I seem to have bad luck with that..). I'm thinking higher plate voltage and maybe a few tweaks here and there but I really enjoyed that amp. With a few pedals you can cover just about any territory you want.

I also need to add DC filament stuff to my 5C3 Deluxe build. The 6SL7/6SN7 setup gets nice and hummy when the amp is cranked.

[norris2002]

Does anyone have any experience with the ebay seller Solsound? they seem to supply a large range or mojotone parts cheaper than allparts stock them. Also, who makes better transformers: mojo or hammond?

[M Fowler]

Hard to say which transformer is better Mojo Vs. Hammond.

I forgot to post this information in another thread, mojo consumer catalog states" Mojo transformers are Mojotone transformers they might come from anywhere in the US. Currently Mojotone transformers are made by Heyboer, Lenco, and Schumacher.

This is their 2009 catalog.

Mark