Buying parts for a D build?

[exocet]

Memory leak here, but either Mouser or Newark are attached to s popular UK vendor (anyone?) that may have stock and presence in OZ.

Big help, I know.

Newark

http://www.farnell.com/

Farnell did have a presence in Oz when I lived there for a while many moons ago but when I recently looked at their 'mouser' sourced components it was much cheaper for me to buy from Mouser directly myself. This option probably only helps users who have existing trade accounts with Farnell and for whom setting up a trade account with Mouser is not feasible. If you are paying by credit card, why bother?

[DonMoose]

I was thinking that Farnell's (thanks!) presence might help with the shipping if they stocked useful parts in useful locations.

I've been in the position of having to order from Farnell into the US to beat some lead time requirements. Far rather do that than go through a gray-market broker.

[exocet]

I was thinking that Farnell's (thanks!) presence might help with the shipping if they stocked useful parts in useful locations.

I've been in the position of having to order from Farnell into the US to beat some lead time requirements. Far rather do that than go through a gray-market broker.

If I look at farnell pricing in the UK for mouser sourced parts, they levy such a high shipping charge that it really isn't worth it. To me that says that they are not stocking those components locally much to my frustration.

[pamaz67]

To tell you the truth one of the most fat and rich amps I've made so fare has been made strating justo from a SOB.
It's true BTW that I'm not usually sticking perfectly at the nominal values in the reference drawings, but i usually tune by ear case by case
Ciao

Paolo

[Mark]

Dear Paolo

Which Dumble amp did you build into the SOB?

What features did you build into the amp (Dumbleator, reverb etc)?

Do you have any pictures of your SOB rebuild?

Why did you find you needed to move away from the intended Dumble circuit?

[rogb]

Mark, I have ordered plenty of parts from Mouser in the US. I'm in England and if you can get everything you need in one shipment, then the cost should be reasonable.

Mouser are very helpful, even when I ordered the wrong part, they sent the replacement straight away.

Just triple-check your order, seriously mate

It's when you forget that one damn part...

I have been bailed out by kind forum members over here.

It's not worth paying the shipping on a single RN65

[renshen1957]

Regarding iron I have an old Boogie SOB amp. I will use the iron from that. It is a 50 watt amp and I will have to try and determine if the power transformer could handle being converted to a 100 watt amp, but that is later on.

Hi Mark,

What is the impedance of primary of the Output Transformer? Also On the Power Transformer, how many milliAmps of Current on H.T. (AC) windings. (If the mA aren't sufficient for the power tubes you won't get more power even if your heaters have enough juice to feed the additional tubes.)

Your B+ to the OT and the OT impedance can make the difference as to whether you can convert 50 watt amp to 100 watt. You have some options:

1) It will be easier to purchase purchase a 100 Watt OT
2) If you want the tonality of 100 Watt amp without the little louder volume, use the 50 watt transformer and use 4 power tubes instead of 2. You will still only have 50 Watts, but the tone of the parallel tubes will produce the tone of 100 Watts (more gMx, transconductance.) And if want the tone of a 50 Watt, just pull a tube from each side of the push pull pairs.
3. Increase (like almost double) the B+ but then you will have to purchase caps with that high a DC voltage rating or stack caps to achieve the same goal (could be expensive, either way)

Keep in mind that 100 Watts is just a bit louder than 50 watts. To get twice as loud as 50 Watts you would need 500 Watts. Then again, more efficient speakers would be a faster way to a louder amp.

Before I forgot, check to see if your Power Transformer has enough heater current to support 2 more power tubes in any either case, especially if you want to use EL34 for example.

Best Regards,

Steve

[Homebelly]

[quote="renshen19572) If you want the tonality of 100 Watt amp without the little louder volume, use the 50 watt transformer and use 4 power tubes instead of 2. You will still only have 50 Watts, but the tone of the parallel tubes will produce the tone of 100 Watts (more gMx, transconductance.) And if want the tone of a 50 Watt, just pull a tube from each side of the push pull pairs.
[/quote]

Hi Steve.

I am interested in knowing more about this kind of set up.
I have KOC TUT3 where he talks about this, but my knoweledge isn't really up to fully understanding how it works.
Is it really as simple as installing two more tubes, or is there more to it?
Would I, for example, just follow the schem and layout of a 100watt amp, but replace the OT with a 50 watter?

[Chris333]

... 2) If you want the tonality of 100 Watt amp without the little louder volume, use the 50 watt transformer and use 4 power tubes instead of 2. You will still only have 50 Watts, but the tone of the parallel tubes will produce the tone of 100 Watts (more gMx, transconductance.) And if want the tone of a 50 Watt, just pull a tube from each side of the push pull pairs...

Are you guys talking about using a 50 watt output transformer with 4 power tubes? Unless I read this wrong, it seems like you'd be stressing the OT by drawing twice the current through it that it's designed for...

[Structo]

Not to mention the primary impedance is all wrong.

[renshen1957]

Hi Steve.

I am interested in knowing more about this kind of set up.
I have KOC TUT3 where he talks about this, but my knoweledge isn't really up to fully understanding how it works.
Is it really as simple as installing two more tubes, or is there more to it?
Would I, for example, just follow the schem and layout of a 100watt amp, but replace the OT with a 50 watter?

KOC explained it quite well in TUT3, for those who do not have benefit of a copy:

GM doubling

"In paralleling tubes we effectively create "composite output tubes" with twice the gm, half the internal resistance and twice the current and plate dissipation capacity compared to a single tube. The mu will be the same as for a single tube, so the shape of the transfer curve should be the same since the new load is half that for a single tube. The input capacitance of the composite tube is twice the value of a single tube, which may load the driver stage at higher frequencies." The extra pair provides the tone of a 100W amp but only 50 watts of power is available.

To answer your question, another way of putting this would be: build a 50 Watt amp, use parallel tubes (for the benefits noted above) and a 50 Watt output transformer to have the aural benefits (the effortless power sound of a 100 watt)

Rather than as you stated, build a 100 watt amp and use a 50 Watt transformer.

You would find KOC's chapter on gMX in tut 4 very enlightening on the subject, but its $100 USD. Well worth the price of admission IMO.

You might want to check out powerscaling.com. KOC is a member and answered a number of questions, when he finds time is beyond my comprehension.

Are you guys talking about using a 50 watt output transformer with 4 power tubes? Unless I read this wrong, it seems like you'd be stressing the OT by drawing twice the current through it that it's designed for...

The maximum power available from any tube amp is determined by the transformer set. An output transformer has a design impedance to match the load to the tubes; it is mildly affected by the tube compliment but is not specifically dependent on the tubes.
The impedance of the output transformer restricts current flow, so the OT won't be drawing twice the current than what it is designed for.

TUT 3 is going on 10 years, with a number of builders who have built parallel power tube versions Plexi (5100) and 800 without OT failures ever being reported.

Not to mention the primary impedance is all wrong.

You can't have a wrong primary impedance on OT on a specific set of tubes. You are limited to the Power transformers ability to deliver so much plate current, at so many milliAmps, and at a tubes plate dissipation in Watts. So the limiting factor is really the transformer set.

In short it is a balancing act.

Using hammond OT transformer specifications (as they're less secretive than some companies), I can build a push pull power amp 15W using 2 6L6 tubes with either 1615 or 1650E which are 5k primary and 8k primary, respectively and the PT's Voltage Amps are both 42. The difference is that the previous OT has 270VDC on the OT transformer and the latter has 340VDC. I would still only have 15 Watts if I went to 6V6 tubes or if I had the heater current to EL34 tubes.

The different PTs in both cases are limited by the 42VA capability, however the higher Volts DC on the 1650E compensates for the higher primary of the OT.

The original Vox AC 30 prototype was a 30W amp with 2 EL 34s. Dick Denney switch to parallel EL84 on the same original transformer set (OT Radio Spares) on the AC30/4 production models for tonal reasons to match the earlier AC 15 sonic signature. The Radio Spares OT winding used in each case was 6600 ohms. The later AC30/6 used about 4300 ohms.

There is no reason one couldn't have a 4 tube 25 Watt amp using any type of octal power tube (with sufficient heater current) with a power transformer rated for 176 VA at 388DC on the plates (typically) and 316 mA DC current with a OT 4300 impedance.

However, with the same OT transformer, Jim Kelley had 60W with 4 6V6 tubes. Higher Plate Voltage was the difference.

One could increase any power Amps wattage by double by increasing the supply rail by 1.414 with the same impedance on the OT, but then you get into transformer Flyback issues on on some tubes during the B phase of AB push pull. However, 622 VDC on the plates isn't everyone's cup of tea if attempting to produce a 200 Watt Dumble ODS. (Significant increase in cost with filter capacitors, too.) There are simpler ways. Besides your neighbors may not love you if you built one.

Best Regards,

Steve