I bought a HiFi amp PCB and I want to make it a guitar amp

[megatrav]

I was browsing eBay (after a few beers) and decided to buy an EF86 EL34 SE amp board.

I am sure that I could use a preamp into it and it would sound good, but I think it might be more fun to build it as a guitar amp.

I am still learning about reading schematics, so I am hoping to get my ass educated by you kind and smart folks.

Here is the schematic:






And the board layout




I am interested to hear input on it!

 

[hamerfan]

I think most of your work will be in the preamp section, but i am not familiar with this ef86 pentode.
Vox used the ef86 for guitar amps. http://www.voxshowroom.com/uk/amp/ac15h1tv.html

 

[fig]

Have you chosen a PT or OT?

You'll be working with deadly current, so please use extreme caution and discharge caps FULLY before poking around.

Mind the component minimum voltages.

Edit: Be sure to get a good heat sink for the LT1085...I'm guessing you'll use the TO-220 package,

 

[megatrav]

Have you chosen a PT or OT?

You'll be working with deadly current, so please use extreme caution and discharge caps FULLY before poking around.

Mind the component minimum voltages.

Edit: Be sure to get a good heat sink for the LT1085...I'm guessing you'll use the TO-220 package,

Thanks for the tip Fig!

I haven't bought anything for it yet. I only have the PCB. I wanted to get a plan together before I bought anything.

I should assume that buying transformers designed for guitar amplifiers would help?

Someone on a Facebook group I am in mentioned the negative feedback, so I am looking into that.

Maybe @vigilante398 our local valve guru might have some input?

 

[vigilante398]

Maybe @vigilante398 our local valve guru might have some input?

In my limited experience (I politely decline the title of guru, I'm too young) the main difference between a hi-fi tube amp and a guitar tube amp is the output transformer, that's what really determines the frequency response of what hits your speaker. Hi-fi transformers are excellent if you're building a bass amp, but for guitar you want something that's going to chop off those lows. There are some component changes that will affect frequency response as well, again the main target is going to be chopping off some lows.

That preamp already has negative feedback, but personally I would take it out, it's a tonal preference and I don't care for it.

Things I would change if this was me taking on the project, in no particular order:

• increase R1 to 68k
• omit C1, C2, and C3
• omit R5 and R6
• Increase R2 to 1M
• omit C4
• increase C5 to 100nF
• decrease C6 to 22nF or less (I wouldn't go lower than 2n2, but tailor to taste)
• Replace the voltage divider at R2/R8 with an A1M pot as a volume control

There are a couple optional changes in the power supply if you want to save a couple bucks. Hi-fi amps tend to filter the absolute hell out of the power, which is fine and doesn't hurt anything, but it's overkill for guitar amps in my opinion. So what I would probably do for simplicity and frugality:

• Omit C13, C15, and C17
• Drop C12, C14, and C16 to 22uF
• you can leave the choke in if you want, or you can take it out and jumper that bit if you want, you would likely be fine either way
• they're running the EL34 heaters straight off the 6.3VAC tap but rectifying it so the EF86 gets 6.3VDC. You should be just fine running the EF86 on AC, which both Vox and Matchless do in their amplifiers.
  • omit B40, C18, C19, IC1, C21, R13, R14, and C22
  • connect the EF86 heaters in parallel with the EL34 heaters

Again this is just what I what do if it were me building it, so there's my $0.02

And I will of course second Fig's advice to BE SUPER CAREFUL around high voltage. It can kill you and it will hurt the whole time you're dying.

 

[hamerfan]

Yeah, that looks a plan: away with the snubber caps and the excessiv filtering. After that the preamp section should look similar a vintage Vox AC15.

 

[megatrav]

Yeah, that looks a plan: away with the snubber caps and the excessiv filtering. After that the preamp section should look similar a vintage Vox AC15.

That would be ideal!

 

[megatrav]

In my limited experience (I politely decline the title of guru, I'm too young) the main difference between a hi-fi tube amp and a guitar tube amp is the output transformer, that's what really determines the frequency response of what hits your speaker. Hi-fi transformers are excellent if you're building a bass amp, but for guitar you want something that's going to chop off those lows. There are some component changes that will affect frequency response as well, again the main target is going to be chopping off some lows.

That preamp already has negative feedback, but personally I would take it out, it's a tonal preference and I don't care for it.

Things I would change if this was me taking on the project, in no particular order:

• increase R1 to 68k
• omit C1, C2, and C3
• omit R5 and R6
• Increase R2 to 1M
• omit C4
• increase C5 to 100nF
• decrease C6 to 22nF or less (I wouldn't go lower than 2n2, but tailor to taste)
• Replace the voltage divider at R2/R8 with an A1M pot as a volume control

There are a couple optional changes in the power supply if you want to save a couple bucks. Hi-fi amps tend to filter the absolute hell out of the power, which is fine and doesn't hurt anything, but it's overkill for guitar amps in my opinion. So what I would probably do for simplicity and frugality:

• Omit C13, C15, and C17
• Drop C12, C14, and C16 to 22uF
• you can leave the choke in if you want, or you can take it out and jumper that bit if you want, you would likely be fine either way
• they're running the EL34 heaters straight off the 6.3VAC tap but rectifying it so the EF86 gets 6.3VDC. You should be just fine running the EF86 on AC, which both Vox and Matchless do in their amplifiers.
  • omit B40, C18, C19, IC1, C21, R13, R14, and C22
  • connect the EF86 heaters in parallel with the EL34 heaters

Again this is just what I what do if it were me building it, so there's my $0.02

And I will of course second Fig's advice to BE SUPER CAREFUL around high voltage. It can kill you and it will hurt the whole time you're dying.

Quick follow up- where is IC1?

 

[megatrav]

I think I may have found the transformers I want:
Hammond 290XXA power transformer
Hammond 1750B Output Transformer

I’m not sure about the choke. I feel like I’m missing the specs for it?

 

[vigilante398]

Quick follow up- where is IC1?

 

[vigilante398]

I think I may have found the transformers I want:
Hammond 290XXA power transformer
Hammond 1750B Output Transformer

I’m not sure about the choke. I feel like I’m missing the specs for it?

Power transformer would be fine, it supplies more than enough current for this amp. You'll be pulling about 1.7A from the heater, transformer is spec'd for 3A, and you're pulling about 70mA from the high voltage, transformer can supply 170mA. So you may be able to find something smaller that would work, but I would expect this to work just fine.

Take another look at output transformers. The 1750B is spec'd for a 5.5K primary impedance, which is too high for EL34, which has a plate impedance of 3K. Most EL34 output transformers are around 3.3K, which is fine, but I 5.5K is definitely not going to give you ideal results.

 

[megatrav]

Power transformer would be fine, it supplies more than enough current for this amp. You'll be pulling about 1.7A from the heater, transformer is spec'd for 3A, and you're pulling about 70mA from the high voltage, transformer can supply 170mA. So you may be able to find something smaller that would work, but I would expect this to work just fine.

Take another look at output transformers. The 1750B is spec'd for a 5.5K primary impedance, which is too high for EL34, which has a plate impedance of 3K. Most EL34 output transformers are around 3.3K, which is fine, but I 5.5K is definitely not going to give you ideal results.

Thank you! I’ll look around for a different one

 

[HamishR]

In general for small amps I don't care for NFB. In the 5F11 Vibrolux removing the NFB was a revelation and I would never bother with it in that circuit. But in the 6G3 Deluxe I just built I put the NFB on a switch, and I'm glad I did. At lower volumes the NFB off is wonderful - the amp is much livelier and fun. But crank it and it becomes very ragged - switching in the NFB makes it sound much better. So if your amp gets a bit too nasty when cranked consider trying a small amount of NFB.

 

[megatrav]

Power transformer would be fine, it supplies more than enough current for this amp. You'll be pulling about 1.7A from the heater, transformer is spec'd for 3A, and you're pulling about 70mA from the high voltage, transformer can supply 170mA. So you may be able to find something smaller that would work, but I would expect this to work just fine.

Take another look at output transformers. The 1750B is spec'd for a 5.5K primary impedance, which is too high for EL34, which has a plate impedance of 3K. Most EL34 output transformers are around 3.3K, which is fine, but I 5.5K is definitely not going to give you ideal results.

I have been busy, but I am still looking into this build.

One of these might be the best option

https://edcorusa.com/products/gxse1...utput-transformer?_pos=3&_fid=d2f51d3ea&_ss=c

https://edcorusa.com/products/xse10...utput-transformer?_pos=5&_fid=d2f51d3ea&_ss=c