Make a Hiwatt 20w with 4 preamp Input

[rafyc]

Hello,

I have a project to make a Hiwatt DR20 clone and have few questions.
But i cant find any diagram ou wiring scheme. Only the Custom 20 here

Ideally i would like to have the same than DR20, 4 preamp input and 0.5W switch.
Could i use the preamp circuit of the DR504 and the power circuit of 20W or it need to revise the whole circuit ?
What about this 0.5 power switch ? how does it works ? is it a switch to Push pull to Single ended ?

I understand that certain types are crucial to have the original sound.
Regarding the transformers I contacted Heyboer Transformers which seem top made from the Partridges and a little more current to be able to integrate a bias adjustment. Only concern is that it is in the US. Tube amp doctor also offers it, but without any details so I don't know the quality.
There are only those of 50W but nothing for 20W and two EL84s.
Do you know who manufactures the transformers for Hiwatt in the UK and what could be the specifications of the transformer for 20W ?


Wanted: Does anyone have a picture of the inside of a DR20 head in good quality?

Thank you !!

[mhuss]

Hylight used the same preamp in every amp they made, 50 to 400 watts, so it would certainly work in a 20 watt. If you use EL84 outputs, you may need to attenuate the PI signal a little (due to their slightly higher gain).

I am not aware of how the Doncaster Hiwatt folks do their real low wattage implementation, turning off one output is certainly an option.

--mark

[rafyc]

Hi Mark,

Thanks a lot for your answer. What do you mean by PI ? sorry English is not my native language.
So good news for the preamp stage ! that said there is 3 version of the 4in Preamp : Early mid and late 70's. Is there a difference in sound ?

Concerning the transformer does a DR504 one could fit for a 20W / 0.5W amp or I need to match it ?

There is one output what do you mean by turning off on output ? that said I'm not sure I could just unplug a tube because there is only two.

Raphaël

[ChopSauce]

PI means "inverseur de phase" (Phase Inverter). It is the ultimate pre-amp stage and no, you can't use anything but transformers dedicated to some "20W" amp, for the power tubes are crucial, both in terms of required (current, voltage) for the power transformer and of reflected impedance (& power handling) for the output one.

So what you seem to currently need most are the specs of the transformers or suitable replacements - maybe from some kind of 18W Marshall amp?
(any guess about the voltages within the amp, at least?)

Note: that 20/0.5W on the standby switch makes me think of some voltage switching "à la"VVR.

[_Steve]

My suggestion for the output transformer would be a quality Hi-Fi one like from the Dynacos (ST35?) - just wire it in Pentode-mode rather than Ultra Linear. There should be some Dynaco equivalent in Europe.

Using an OT designed for a Marshall 18W might saturate too early and not have the wide frequency response that the Partridges have (if you believe that is important).

[rafyc]

PI means "inverseur de phase" (Phase Inverter). It is the ultimate pre-amp stage and no, you can't use anything but transformers dedicated to some "20W" amp, for the power tubes are crucial, both in terms of required (current, voltage) for the power transformer and of reflected impedance (& power handling) for the output one.

So what you seem to currently need most are the specs of the transformers or suitable replacements - maybe from some kind of 18W Marshall amp?
(any guess about the voltages within the amp, at least?)

Note: that 20/0.5W on the standby switch makes me think of some voltage switching "à la"VVR.

Hey ! you there
Ok thanks for the explanation. I think that PI est crucial for the Hiwatt clean sound.
I've sent a mail to Hiwatt saying my amp blown up to know if they have spare parts. I've ordered to them fe years ago Custom Fane you cant buy elsewhere.

Concerning the Switch here is a bad quality picture from youtube of a Littl J rig, also with this switch.


I've not a lot of technical background so if someone could help trying to recognize something on this Pixel soup it will help. Do you think it's VVR ? i can't see any Mosfet transistor.

[rafyc]

My suggestion for the output transformer would be a quality Hi-Fi one like from the Dynacos (ST35?) - just wire it in Pentode-mode rather than Ultra Linear. There should be some Dynaco equivalent in Europe.

Using an OT designed for a Marshall 18W might saturate too early and not have the wide frequency response that the Partridges have (if you believe that is important).

Hi Steve, thanks for your answer but i'm trying to stick the most of the original Hiwatt.

[ChopSauce]

A la VVR, NOT a VVR. in extenso: voltage switching on either side of the standby switch, for example one side provides the usual (20W) power output, while the other side starves the output tubes with a (much) lower voltage. That would be consistent with the amp providing just an ON(20W)/OFF(stby)/ON(0.5W) switch and not a pot, like any VVR would.

Hammond also provides a line of HiFi output transformers, which are easier to buy from France yet pretty expensive too.

[rafyc]

Super useful informations ! thanks
Ok for the switch but how would you lower the voltage the output ? with Zenher Diodes ? no overheating ?

I found a transformer for the 20w Head
It refers as "Mains" i assume it's an input transformer ? and Mercury have an output transformer for 20W Hiwatt but much more expensive.
Should i take the same brand for the transformer or i could go with two different ?

Does heyboer still making transformers ? Site seems to be down, no shop and dont answer on FB.

[ChopSauce]

Mhh... not quite sure about the transformer...

Maybe it's ok but 250V doesn't seem so much. I almost don't know anything about Hiwatt amps but I thought they have beefier transformers. So, I could have been be more tempted by - see: http://primarywindings.com/amp-maker/ -

mainstxframe01.jpg

and maybe a 18W transformer is not Ok, but their seems to be almost the same size as a Hammond from their HiFi line of products https://www.hammfg.com/electronics/tran ... /1608-1650

Note: you should take care of the reflected impedance and still you do not seem to know much about the voltages inside the amp. That would work but how close to the real thing?

Note2: by switching the 0 or the 190V windings you can supply either 275 (275-0) or 85 V (275-190) to the rectifier - if I'm not wrong with the maths - which would do the power reduction (yet unlikely in the proportions claimed by Hiwatt... )

[_Steve]

My suggestion for the output transformer would be a quality Hi-Fi one like from the Dynacos (ST35?) - just wire it in Pentode-mode rather than Ultra Linear. There should be some Dynaco equivalent in Europe.

Using an OT designed for a Marshall 18W might saturate too early and not have the wide frequency response that the Partridges have (if you believe that is important).

Hi Steve, thanks for your answer but i'm trying to stick the most of the original Hiwatt.

OK no problem, but just be clear that you will be trying to recreate a recreation. By that I mean that the *new* Hiwatt brand is trying to recreate the classic amps but in a 20W package - which never existed in the 70s/80s. I don't know if they were successful in recreating it well or not, but the original 50W, 100W and 200W Partridge transformers were differentiated by being over-sized and having a wider frequency response than regular guitar-amp transformers. It is claimed that they were in fact originally designed as HIFI OTs. That's why I suggested looking for a good HIFI OT rather than a guitar one. The company now operating with the Hiwatt brand may or may not have specced appropriate OTs, but in any case I doubt you will be able to obtain that spec from whoever supplies them due to exclusivity/confidentiality agreements.

[mhuss]

The advantage of being a manufacturer is you can have transformers built to spec. A power transformer with dual secondaries would let you switch between two widely different power supply voltages, a 'normal' one and a much lower one.

Unfortunately, I don't imagine that a HALFWATT would sound very Hiwatt-like. As Steve mentioned, high headroom and wide bandwidth are a key part of the HIWATT sound.

> Two tubes
You could switch one output tube input between normal and ground. In the ground position, that tube would just conduct a balancing 'idle' DC current which would keep the output transformer happy, but the impedance mismatch and lack of tube signal would make the output much smaller.

[jazzbass]

Hi crafyc,

take a look at the Tubetown ttac18 kit, the wiring diagram shows a very effective system to obtain extremely low output power, I don't know exactly how many watts but it is at bedroom level, I used this kit and it works perfectly.
I also want to build a Hiwatt using the Custom 20 scheme, and I will do so using part of the components supplied with the Triwatt kit (which you can find at Trinityamps.com) with which I built an amp, I think I will use two 6V6s as power tubes and the transformers supplied with the kit.

A hug

Franco