DIY Valve Guitar Amp Head

[DrNomis_44]

Update:

Been seriously thinking of switching from two EL34 power tubes to two 6L6 power tubes, I think the Soldano SLO 100 originally came from the factory with four 5881 power tubes, but I could be wrong, the 5881 has the same internal electrode design as the 6L6.

[Marcel]

Swapped the EL34's for a pair of 6L6's in my 2204 build... I figured as it was originally built with 6BQ5's, and modified to 6L6's before I got it then it would probably be a good fit.

While now with the 6L6's the output impedance selector works correctly the output volume level is noticeably reduced. With the EL34's there was a lot more volume and "grunt" from the amp but the impedance selector needed to be in the next position for the amp to 'behave'. That's not to say the 6L6's are weak, hell no, they do kick out substantial clean volume, maybe I'm just noticing the absence of 'dirt'... As HT volts, biasing, output transformer ratios and drive signal size all play important parts I'm guessing the build went from a solid and gritty 50W to a (for the moment) squeaky clean 40W...

I can't call it a 2204 build any more so its new name after it gets the EF86 input stage upgrade is the 6682 build... It will be interesting to hear how clean things get before further rounds of modifications to 'beef it up' again...

[DrNomis_44]

Swapped the EL34's for a pair of 6L6's in my 2204 build... I figured as it was originally built with 6BQ5's, and modified to 6L6's before I got it then it would probably be a good fit.

While now with the 6L6's the output impedance selector works correctly the output volume level is noticeably reduced. With the EL34's there was a lot more volume and "grunt" from the amp but the impedance selector needed to be in the next position for the amp to 'behave'. That's not to say the 6L6's are weak, hell no, they do kick out substantial clean volume, maybe I'm just noticing the absence of 'dirt'... As HT volts, biasing, output transformer ratios and drive signal size all play important parts I'm guessing the build went from a solid and gritty 50W to a (for the moment) squeaky clean 40W...

I can't call it a 2204 build any more so its new name after it gets the EF86 input stage upgrade is the 6682 build... It will be interesting to hear how clean things get before further rounds of modifications to 'beef it up' again...

Did you use the same output transformer you had in it when you swapped the stock tubes for 6L6s?, if so, that might account for the perceived loss in output level, since the plate load for an EL34 is different to what the plate load for 6L6, that's probably why the impedance selector worked properly with the 6L6 tubes.

[Marcel]

Yep, same transformer... Tough they sounded fine it was impedance issues with the EL34's that gave the main reason for switching back to 6L6's...

[DrNomis_44]

Yep, same transformer... Tough they sounded fine it was impedance issues with the EL34's that gave the main reason for switching back to 6L6's...

Ah okay, yeah that would do it, power tubes are more picky about the load impedance they're working with than solid-state devices.

[DrNomis_44]

Update:


Thought it was about time I did an update on this thread, okay, so earlier today I ordered some things from Evatco, a new Accutronics reverb tank for my Marshall MA 100C amp, two Bourns 500k A 24mm pots, two stereo Cliff Jacks (for my Fender Super Twin amp), and ten 28X2 tagstrip boards which I will be using to mount all the components in my SLO-50 amp build, I'm going to be calling the amp the Sloclone 50.

Here's what the tagboards look like:

https://www.evatco.com.au/tag-strip-28-x-2


I've elected to use these 28 X 2 tagboards because it looks to me the simplest way to build the circuits, you just solder the resistors and caps between two adjacent tags, and link the tags together where they need to be linked, of course I'll need to work out a suitable layout of the components on the boards beforehand, using some 5mm square graph paper, but at least I won't need to buy any special tools, the insulation on those 28 X 2 tagboards looks to be FR-$ Fibreglass similar to that used to make printed circuit boards, so the dielectric strength should be fine, the highest voltage in the amp is most likely going to be about 498V DC on the centre-tap of the output transformer, with about 384V DC going to the majority of the preamp and phase-splitter circuitry.

Dielectric-strength is basically a measure of how many volts that a piece of insulation material can withstand before it suffers an insulation-breakdown and leaks DC which is particularly important in valve amplifiers where the anti-phase outputs from the phase inverter are coupled to the grids of the power valves with high-voltage non-polarized caps, typically something like the pair of 22nF (.022uF)/630V DC caps that you find in Marshall amps like the Superlead Plexi, you want the dielectric, or insulator in the cap to block DC and pass only AC, because any DC leakage to the grids of the power valves will upset the biasing and cause the power valves to conduct too much current, when that happens the power valves can start to red-plate and glow cherry red or orange, or in some cases, the plate can literally melt because it is only made from thin metal sheeting and too much current makes it heat up very hot, that's one way to destroy a power valve.


Here's a question for you, Marcel, on the Evatco website they say that the Gold Lion KT-77 Power Valve that they sell is a drop-in replacement for an EL34 or 6CA7, my valve amp head build uses an output transformer designed for two EL34 power valves, if I put two of those Gold Lion KT-77 valves in the sockets will the output transformer work fine with them or would I need to swap it out for one designed for KT-77 power valves? I'm assuming that the existing output transformer should work fine.

Here's the Gold Lion KT-77 page on the Evatco website:

https://www.evatco.com.au/genalex-gold-lion-kt77

[DrNomis_44]

Update:


Just found out that Evatco do stock the optocouplers I need to implement the channel switching, so, I'm going to go with the original optocoupler channel switching system rather than use relays.

[DrNomis_44]

Update:


Just got a parcel from Evatco delivered a few minutes ago, inside it were a new Accutronics reverb tank I had ordered for my Marshall MA100c amp, two Bourns 500kA 25mm pots, two stereo Cliff Jacks, and ten of these 28 X 2 Tagboards I ordered for my diy scratch-build Soldano SLO 50 Sloclone 50 amp build, these will be used for mounting all the parts in the amp.

[DrNomis_44]

What was the rough cost of parts Doc?

Just did a quick re-assessment of how much the amp parts have cost me so far, and for a rough estimate I've probably spent less than Au$1,000.00 on it so far, mostly because some of the parts I didn't actually pay any money for, like the 2mm Aluminium sheeting I used to make the chassis, I scrounged the iron for the power transformer from an old movie projector, or a reel-to-reel tape recorder, my memory is a bit rusty, the only parts I had to buy were the 50W output transformer, which cost me something like Au$350.00 or so, maybe a bit more, plus the various resistors and etc, a friend of mine gave me two LCR dual 50uF/500V caps for nothing, I wished I had kept records of what parts I did buy so I could make a more accurate assessment.

[DrNomis_44]

Update:

Managed to get my HP Officejet printer working again, and found some spare A4 printer paper, so, I went and printed-out all the Sloclone schematics and Turret Board layouts I have collected, tomorrow I'm going to start working on making some layout diagrams for the 28 X 2 Tagboards I'll be using to mount all the components, after that, I'll start prep-work on the tagboards so they will be ready for populating with components.