5F8-A HIGH POWER MOD FOR BASS

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DaveWell
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5F8-A HIGH POWER MOD FOR BASS

Post by DaveWell »

Hi !

I know the TWEED TWIN 5F8-A HIGH POWER is a good amp for guitar and bass
I'm looking for[u] modification[/u] in the layout to make it MORE for a Bass/guitar
Gonna built it with only one input
and only the normal channel
I plan to use 4 x 6550 tube amp insted of the 6L6.

I need Help on this guy's !

Thanks !
Stevem
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Re: 5F8-A HIGH POWER MOD FOR BASS

Post by Stevem »

What have you built before ?
The use of 6550's will get you no more ultimate power, just a tad more clean head room and cost you 50 bucks more in new tubes, but heck it's your Wallet!
For use with just Bass I would ditch the number 83 tube recto ( there hard as Hell to find now anyway! ) and run SS .

This will kick up your plate voltage from about 390 to 420 which will be good for punch and tightness, but if you want to keep the driver and preamp section sounding as stock then that 4700 ohm power supply dropping resistor will need to go to about 5200 ohms and I would double its wattage rating also.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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didit
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Re: 5F8-A HIGH POWER MOD FOR BASS

Post by didit »

Quad of 6550 (or KT88) requires big iron and more. A 5F8-a preamp with tweaks could work.

Suggest looking through these two build threads for ideas: Best .. Ian
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DaveWell
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Re: 5F8-A HIGH POWER MOD FOR BASS

Post by DaveWell »

Thanks for the suggestion.

Stevem,i have built 4 Express already, and 2Stroke amp and a 1973 superbass rebuilt.
I will check over EBay for a 83 rectifier, but you say Rectifier tube + SS rectifier or only SS rectifier ?

The Iron Will be Mercury Magnetic OT PWT and a choke.

I will beef up the 2 first 20/20uf cap for a 100UF (50/50) the second up tu 47uf and the preamp 22uf

Would it make sence to beef up the V1 12AY7 pin 3 from 250uF to a 330uF ?

Someone have other mods suggestion ?

regards
Dave
sluckey
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Re: 5F8-A HIGH POWER MOD FOR BASS

Post by sluckey »

5F8-A uses a GZ34, not an 83. The older 5F8 does use an 83. If you intend to use this with Bass and 100µF filter caps then you really want to go with SS rectifier.
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DaveWell
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Re: 5F8-A HIGH POWER MOD FOR BASS

Post by DaveWell »

Ceriaton add 2 diode on the GZ34

What you think about it ?

Will this be a good alternative...
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pompeiisneaks
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Re: 5F8-A HIGH POWER MOD FOR BASS

Post by pompeiisneaks »

a GZ34 max capacitor connected to it is 60Uf, you don't want to push that with a 100uF as mentioned. It's better, if you want stronger filtering (which is also better for a bass amp) to just use SS rectification.

~Phil
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Stevem
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Re: 5F8-A HIGH POWER MOD FOR BASS

Post by Stevem »

Whatever the unloaded AC output of your chosen PT is here's the conversion for rectified D.C. Output.
By means of tube like a 5AR4 AC voltage X 1.3
For SS diode , AC X 1.4.
Since your dropping off the tube for what was the other channel you will have some free space so you might concider running 6 6L6 output tubes at a nice cool bias and they will sound cleaner and live a very long life, but then again you could run two KT 120s!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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DaveWell
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Re: 5F8-A HIGH POWER MOD FOR BASS

Post by DaveWell »

ok i definitly go for a SS rectifier,

2 other propose mod by a freind

Input 27K insted of 68K

Vi cathode 1K/68uF , insted of a 820r/250uF

What you guy's think about that ?

Thanks
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Re: 5F8-A HIGH POWER MOD FOR BASS

Post by pompeiisneaks »

DaveWell wrote: Thu Nov 23, 2017 2:39 pm ok i definitly go for a SS rectifier,

2 other propose mod by a freind

Input 27K insted of 68K

Vi cathode 1K/68uF , insted of a 820r/250uF

What you guy's think about that ?

Thanks
That .68uF will cut out on some bass response. You'll want the 250uF normally (thus why they used it on the bassman instead of the more common 10-25uF on other fenders etc.) Unless I'm misreading that and you meant 68uF? basically the lower the uF the lower the frequency response. You could meddle with that value a bit, but there are online calculators like this one:

http://www.learningaboutelectronics.com ... hp#answer1

that help figure out the optimal values for your needed cutoff. In this case a 6 string bass with the low B string being around 30.8hz, means you could get away with something like 1k and about 8uF and still get the low end you need without muddiness. (that makes the low end cutoff at 20hz giving you a bit of room, but it's up to you) that 820 vs 1k on the cathode just adjusts the bias, if I recall 1k is pretty middle of the road, and will probably be nice for bass, but so would 820, 1.5k pushes it to a bit more drive, so may not be best for bass.

~Phil
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DaveWell
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Re: 5F8-A HIGH POWER MOD FOR BASS

Post by DaveWell »

Hey Phil,

the information are from Rooster, here what he wrote:
For example, the typical 1.5k/25uf will go down to 160-180hz, pretty much the freq of a low E guitar string. Which is to say that that 250uf cap will do 25hz or so - worthless - a fart machine using a bass. The 50-63uf is plenty fat and tighter overall. Some builders actually use 25uf for bass guitar, be advised.

And for the input resistok 68K do down to a 27K ? Any infos on that ?

Dave
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drew
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Re: 5F8-A HIGH POWER MOD FOR BASS

Post by drew »

Can I ask why you don't want to just build a proven bass amp? It seems like you're making things harder and more uncertain, when they don't need to be.
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DaveWell
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Re: 5F8-A HIGH POWER MOD FOR BASS

Post by DaveWell »

Hi Drew,

Cause my freind whant a Bassman 100W , but he whant it more adapt for bass...

Sure I was thinking about a vintage Bass amp... but he already have a Ampeg SVT...

Thanks
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DaveWell
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Re: 5F8-A HIGH POWER MOD FOR BASS

Post by DaveWell »

Hey !

I check at the Ceriaton Bassman clone and they add a 22K before the 10Uf 160V..

Why is that for ?
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sluckey
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Re: 5F8-A HIGH POWER MOD FOR BASS

Post by sluckey »

It sets the low end of the bias pot adjustment and will prevent you from setting the bias voltage to zero and burning up your power tubes. The original circuit had a 56K resistor in that position and was not adjustable. Ceriatone chose to provide adjustable bias so they replaced that 56K with a 25K pot and a 22K resistor.
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