Standel 25L15 OT

[Leevi]

What is the recommended primary impedance of the OT of Standel 25L15 (in picture)?
6.6K is proposed in the schematic but is it correct?

/Leevi

[PRR]

Seems reasonable. 8K is another option, not much different. Standard 4K is too low for this.

[Leevi]

Thanks PRR.


How about the size of the iron?
Is a big size essential in this amp?


/Leevi

[PRR]

Use Hi-Fi 25W-35W.

[Leevi]

Use Hi-Fi 25W-35W

Like the following Hammond?

https://www.hammfg.com/files/parts/pdf/1650H.pdf

/Leevi

[sluckey]

May I suggest the 1650HA instead? It's the same OT but it doesn't require a code book to wire the secondaries.

     https://www.hammfg.com/part/1650HA?referer=1047

[Leevi]

May I suggest the 1650HA instead? It's the same OT but it doesn't require a code book to wire the secondaries.

That's fully true especially if you want to use all secondaries.


/Leevi

[Leevi]

Check always the pin numbering! In the schematic I sent the Grid #1 and #2 are wrongly numbered.


/Leevi

[DummyLoad]

very nice sounding OT. highly recommended. i used one of these in the boilermaker amp recently posted. 

https://www.mojotone.com/amp-parts/Output-Transformers/Mojotone-TW35-Project-Output-Transformer

another option is the classictone equivalent - to me the heyboer part sounds a bit better with LF.

http://www.classictone.net/40-18064.pdf
buy it here:
https://shop.amppartsdirect.com/TW-Style-30W-Output-Transformer-40-18064-40-18064.htm


 --pete

[MFowler]

Or call Heyboer and order TWOT 6K6


Or visit Bob at www.rjguitars.net and order Express 6k6 OT

[Leevi]

Thank you for your hints but the amp has already been wired.
I ended up to Hammond 1650H since the 1650HA was out of stock.


The amp sounds clean is very loud.


/Leevi

[DummyLoad]

Thank you for your hints but the amp has already been wired.
I ended up to Hammond 1650H since the 1650HA was out of stock.


The amp sounds clean is very loud.


/Leevi

https://www.tubesandmore.com/products/switch-rotary-impedance-selector-1600-series


rotary switch pre-wired to select output Z without having to rewire the NON-A version of 1600 series OT to change speaker load Z.


--Pete

[DummyLoad]

Thank you for your hints but the amp has already been wired.
I ended up to Hammond 1650H since the 1650HA was out of stock.


The amp sounds clean is very loud.


/Leevi

yes, yes it is... tone controls are interesting. i bonded the two input channels on my breadboard. got some interesting and pleasing grit/grind going with hot humbuckers.


--pete

[Leevi]

https://www.tubesandmore.com/products/switch-rotary-impedance-selector-1600-seriesrotary switch pre-wired to select output Z without having to rewire the NON-A version of 1600 series OT to change speaker load Z.

I'm a bit sceptical about those light rotary switches since their capacity to handle bigger amount of current is limited.
Below is one improved version of 3-pos switch where the all contacts of the switch are used.


https://en.uraltone.com/ut-impedance-selector-switch-pcb-for-alpha-rotary-switch.html


/Leevi

[Leevi]

Which one of the power tube grid stoppers is recommendable?
There are 47K resistors marked in the schematic but I saw in some other sources that 4.7K is used.
/Leevi

[sluckey]

I've only seen one schematic so I would use the 47Ks. I would also send a pm to HotBluePlates because I think he has a lot of info on this amp.

[MFowler]

4k7 is shown on the Weber JP25 which is the Standel 25L15.

[SILVERGUN]

Which one of the power tube grid stoppers is recommendable?
There are 47K resistors marked in the schematic but I saw in some other sources that 4.7K is used.
/Leevi

4k7 is shown on the Weber JP25 which is the Standel 25L15.

It looks like you've pointed out g2 screen grid resistors and he was asking about g1 "grid stoppers"

[Leevi]

Weber seems to use 5.1K.


In the following schematic shared  by HotBluePlates 4.7K is used.


http://el34world.com/Forum/index.php?action=dlattach;topic=14263.0;attach=36312;image


/Leevi

[SILVERGUN]

Weber seems to use 5.1K.


In the following schematic shared  by HotBluePlates 4.7K is used.


http://el34world.com/Forum/index.php?action=dlattach;topic=14263.0;attach=36312;image


/Leevi

This schematic shows 100 ohm Rg1 with a funky, shared 100 ohm tail... how accurate is that?

Either way, I wouldn't sweat the difference between 4k7 and 5k1

[Leevi]

This schematic shows 100 ohm Rg1 with a funky, shared 100 ohm tail... how accurate is that?

Yes, it must be a fault. Also G1 and G2 are wrongly marked.


G1 = pin number 3
G2 = pin number 2


/Leevi

[DummyLoad]

Which one of the power tube grid stoppers is recommendable?
There are 47K resistors marked in the schematic but I saw in some other sources that 4.7K is used.
/Leevi

that "original" 25L15 schematic has a few errors.


they are 4.7K - not 47K


--pete

[Leevi]

I spent lot of time with an issue of the tone stack.

When guitar is plugged in the lower  input and you turn both volumes almost open the amp starts behave like having
blocking distortion. I localized the problem to the tone stack circuit and spent lot of time by rewiring and changing components.

I checked the hand written schematic more carefully and noticed that the 10K plate resistor of the 12AT7 was removed. I removed it as well
which solved the problem.

http://www.prowessamplifiers.com/schematics/misc/Standel_25L15-Standel_25L15.html

/Leevi