25 watt EL34 JCM 800 build

[mehfuzhoss]

Hello,

I found a schematic/layout at Rob robinette's site for this amazing 25 watt design and planning to build it. the layout looks great and should be fun to play since it puts out a little bit low power; so i should be able to crank the amp little more since i live in an apartment. few things i like to ask you for suggestions before i start order the parts for it -

PT OT :
Hammond 1750N - Output Transformer - 3.2K, 4/8/16 OHM, PP, 50W
and
Hammond 290GX - Power Transformer with Primary: 120/220/240VAC 50/60Hz Secondary: 690VCT @ 150mA 6.3VCT @ 5A

will these trannies support 2 el34 configuration as shown in the layout?

does Sozo 500V vs malory 630V make any audible tonal or feel difference ? if so, i will spend the extra on the sozo

also, it has a type 2 master volume which comes handy for a high gain/low vol situation, but I am thinking of a VVR as well. what is your suggestion on this?

*higher resolution layout and schematic link : https://robrobinette.com/images/Guitar/ ... Layout.pdf

Thank you.

[bmx]

To get down to 25W, you want to use a PT with a lower voltage to generate B+. You would calculate the B+ needed and the Primary Impedance of the OPT at the same time. 690V secondary and the OPT you specify will get you 50W, not 25W. Hope that is helpful

[sluckey]

Just a thought... Cranking a 25 watt amp will disturb your neighbors just as much as cranking a 50 watt amp.

[wpaulvogel]

Rob’s amp says 6V6, not EL34’s. The 6V6 version will do close to 25 watts with a little bit different tone but the output transformer has to be mismatched to about 6.4k. The EL34’s are gonna do about 50 watts at 3.2k.

[mehfuzhoss]

I found classictone 40 18086 to be a fit PT.

I might go with a 50 watt and vvr and ppimv combined.

Will think this weekend....

[Stevem]

The problem is that a 6V6 being a beam type tube does not have the gain / dynamics and feel of a EL34 or EL84!

I would build your low watt baby with a power scaling set up from London power.

[goldenGeek]

I have built several 25W-ish amps with Hammond 290NX for power and 1750H/1760H for output, running a pair of EL34

[bmx]

I was just listening to the Tone-Talk podcast interview with Santiago Alvarez, amp designer formerly of Marshall.
https://www.youtube.com/watch?v=Ba4zJHpSeCQ

At the end someone asks about the 25Watt Marshall Studio series. Santiago states that the power amp is EL34 based run at Low B+ maybe even down at 260V. Anyone confirm this ow B+ value for the Power section?

[Schmirleroo]

If you want to build a low wattage 'Marshall' type build the amp with EL84's and use the Hammond 1750PA (8K4 @ 18 watts)
or... if you want to go with 6V6's use the Hammond 1750E (8K @ 15 watts)
EL34's with that PT will be pushing you up into the 40 watt range

[Schmirleroo]

I have built several 25W-ish amps with Hammond 290NX for power and 1750H/1760H for output, running a pair of EL34

Any impedance mismatch problems using that 6K6 OT with the EL34's?
Don't they prefer something down around the 3-4K range?

[frankdrebin]

impedance is a function of tube type,plate and screen voltages,class of operation,type of configuration(pentode,triode,ultralinear).
Amps that use 3400 ohms primary with 2 EL34 at over 450V are all plain WRONG!

[Littlewyan]

The Mullard EL34 datasheet states 3.4K Primary with 425V on the plates. Oddly enough it doesn't give you a screen voltage, but does state the screen resistor should be 1K.

I've just built a 2 x EL34 amp with 474V on the plates and 6.6K impedance OT. Admittedly I thought the voltage was actually going to be 440V, but even at that, I was fine with an 6.6K OT. Worst case I would have just upped the screen resistors to 1.5K or 2K.

[bmx]

Running high primary impedances is maybe a Trainwreck thing. Running low primary impedances is a Marshall thing. What you might want to do is do a loadline for what the B+ is when actually run at full power. We all know that many old Marshalls sagged up to 100V when maxed out. So, do your loadline at Sag, and you can be "Right" with a 3.0K-3.8k OPT. This is off topic though.

I think the best best for 25W EL34 power amp is to use a Vox AC30 OPT and find a Power Transformer that will give around 280VDC, or at least sag to that spot. The AC30 idled quite Hot, so there is room to get those el34's biased to your preference as long as 1xEL34 !> 2xEL84 bias current.

[Ray Barbee]

For the record, the ctone 2202 is a 6v6 based 2204. It both sounds and feels like some of the best 2204s I've played through, and much better than the 2204 builds I've played with 34s and low Vp to drop the power to 25 watts.

Point there is: do not be afraid to build low watt marshall style amps with 6v6s. You will not pine for 34s.