A Rocket born - newbie voltage questions

duriehill · Post by **duriehill** » Sun Dec 29, 2013 8:35 pm

Hello all

I've just about completed my first self made build, and looking for some feedback on queries I have which may also help other 'newbie' folks in future.

I'm try to understand the voltage readings I have (attached)

Initially they seem a bit high to me. Generally the 'B+'s' seem to be 30-40VDC over the expected. However overall this seems to bring it in line with AC30 specs, from research. The amp it loud, and seems to have quite a bit of head room. The clean tone is clear, responsive and chimey. Yum. But the distorted tone I'm not digging. It's fairly gritty - and I suspect this is due to running the amp quite hot?

If I wanted to lower voltages, what's the best approach? Is it a case of adjusting the dropping string resistors? Or could I install a hi-watt resistor post rectifier?

Are there particular voltages I should pay closer attention to to fall within the expected and other's that are fine within a window and are open to taste?

I'm also wary of pushing the tubes too hard..

It's a beautifully quiet amp - I'll post another pic when it's in it's cab (it's zwreck style).

Thanks

martin manning · Post by **martin manning** » Sun Dec 29, 2013 9:30 pm

Since the filament voltage is right on I guess this is not a mains voltage issue. Seems like the PT secondary is just higher than "expected," and everything is about the same 15% or so high. Expected shows 506 on the secondaries, and if you are using Hammond 370HX it's spec is at 550 at 230 mA. Your current draw is not that high at idle, so it'll be higher yet.

You could put a big Zener in the HT CT to drop the voltage down, but you'll have to heat sink it. You could more easily leave the plate and screen voltages where they are and lower the preamp voltages to the expected level by increasing the dropping resistors after the screen node (the two 9k1's).

duriehill · Post by **duriehill** » Mon Dec 30, 2013 11:12 am

Thanks Martin, appreciate the advice.

Perhaps I might look at the zener idea. I'm not sure I understand how a diode is used to drop voltage, but I'll research this. I assume it clips the top off the voltage to reduce.

I think I was getting about 580 on the secondaries unloaded. Do you think a 50v, 25watt zener would be appropriate?

I'm not 100% sure where the zener should sit in my PS, see:
[img:422:287]http://s29.postimg.org/foh03uwc7/my_rectifier.jpg[/img]
At point A?

I like the idea too of increasing the dropping resistors for the preamp values. How much do you think I would need to increase these by - to what value? Is there a formula to calculate this?

Thanks

martin manning · Post by **martin manning** » Mon Dec 30, 2013 1:05 pm

from B+2 to B+3 you are going from 326 to 241 through 18k2, and you want to get down to 212. I'd try 18k2 x (326-241)/(326-212) = 24k4. That assumes constant current, so it's an approximation. A pair of 12k in series or a pair of 25k in parallel would be a good start.

The Zener would go at point A in your sketch. A 25W, 50V Zener could handle 25W/50V = 0.4A (400mA), which is overkill. You can also use a series string of smaller diodes, say 3 or 4 15V 5W and you wouldn't need to use a heat sink.

Here's some info on using a power MOSFET: http://www.geofex.com/Article_Folders/m ... tfolly.htm At the bottom of the page you'll see the conventional version with the Zener between CT and ground as well as the "amplified Zener" with MOSFET.

If you search around you'll find a lot of info on this subject.

njwrecks · Post by **njwrecks** » Mon Dec 30, 2013 3:40 pm

Hi Duriehill- I'm new to the Rocket builds- yours looks great- what kind of cable clamp is that? cool stuff...

duriehill · Post by **duriehill** » Mon Dec 30, 2013 4:15 pm

Thanks! It's not a clone in terms of layout as I had a smaller chassis to work with, but that's been part of the fun. I drew up the layout plans in cad and used a German company to machine the face-plates.

This is the cable clamp I used:
http://uk.mouser.com/Search/ProductDeta ... 46-1427CG9

njwrecks · Post by **njwrecks** » Thu Jan 02, 2014 4:22 am

Thanks duriehill- I just don't like the removable powerchord thing- been trying to find a good clamp...your amp looks great!!

duriehill · Post by **duriehill** » Fri Jan 03, 2014 12:59 pm

Thanks Martin. A friend has a zener that I will borrow to start with.

I'll report back shortly.[/url]

duriehill · Post by **duriehill** » Mon Jan 06, 2014 9:47 pm

right, so I've installed the zener.

But I think I've done something wrong as there is no noticeable voltage drop

(all voltages are the same)

I've placed it between the HT centre tap to ground (see photo).

Any ideas where I've gone wrong??

martin manning · Post by **martin manning** » Mon Jan 06, 2014 10:22 pm

Either it is shorted, or the case of that diode is the anode and you are only getting the forward voltage drop and not the Zener voltage.

duriehill · Post by **duriehill** » Mon Jan 06, 2014 10:35 pm

Ah, I'm tired. That makes sense, the case is probably the anode. I assume one must be able to purchase a reverse polarity where the case is the cathode.

martin manning · Post by **martin manning** » Mon Jan 06, 2014 11:30 pm

duriehill wrote:I assume one must be able to purchase a reverse polarity where the case is the cathode.

Yes, usually with an "R" suffix on the part number for reverse polarity.

duriehill · Post by **duriehill** » Mon Jan 20, 2014 9:48 pm

So I've finally finished after lot's of 'tweaking' and lessons learned. I'd thought I would share some of them, however obvious they may be.

Firstly, attached are the pictures of the final build. I made a few changes to the layout from my first posting.

- Filter caps orientation. You'll notice in the first pictures posted in this topic that the filter caps were installed with the + side facing the turret board. This seemed to introduce hum, so rotated these 90 deg, Ken's way. I found it's important to pay attention to this.

And while I'm discussing the filter caps - what glue do you guys use to mount them to the chassis? I used just a hot melt glue gun, but I'm concerned that after using the amp for a few hours the chassis gets pretty hot and that over time this will weaken the glue or be hot enough to melt it!

- Grounding. Initially I followed the layout of Ken's amp for grounding, but I found it to be quite noisey. This is probably not the case with a chassis size and layout of a real Rocket, but my chassis width is a lot thinner, so I had to veer off the layout. I ended up grounding the heater centre tap at the 1st point at the preamp input, along with the input jack and preamp buss. In the middle of the chassis (centred between the EL84's) was the 2nd gounding point for the phase inverter and screens filter. The 3rd lastly in the
PT corner for everything else (HT CT, earth, BT+, OT, power cathodes). It's now very quiet.

- Zener diodes to reduce B+. As suggested I installed some 5x 5W 10V zeners. This seems to have perfectly brought the voltages to the expected. I've noticed though these guys get pretty warm. They total 25W so this should be more than enough power. I installed them on two tag strips and added extra buss wire at the tags to create somewhat of a heatsink, not sure how affective it is! Do you guys find these zeners reliable?
(See attached picture)

- V1 and V2 hum problems. It took me a while to work out what was going on here. But essentially I learnt that some of the modern preamp valves that I was using are NOT suited to these positions in this amp. Particularly the new Tung Sol's and actually the Sovtek LPS too. They're not suited because of the high cathode-to-heater voltages I understand. I used JJ's and they are quiet as a mouse, and actually sound pretty sweet (I'm not normally a user of them).

- Pre-amp ceramic sockets. Found that one of these guys wasn't making a tight connection across a range of valves. Swapped these out for the belton type.

- Bass pot resistor/wiper. RJ suggested in one of his posts tweaking the tonestack on the bass control. When you max out the bass control the mid's take a huge dive. I removed one end of the resistor from the third leg of the pot and connected it to ground. This leaves one end of the mid resistor connected to the wiper on the bass pot and the other one connected directly to ground.

The amp really sounds special, extremely musical. I'm very happy with it. So thanks all!

duriehill · Post by **duriehill** » Mon Jan 20, 2014 9:57 pm

More pictures..

M Fowler · Post by **M Fowler** » Mon Jan 20, 2014 10:04 pm

Very nice looking build.

I'm really surprised at all the problems since the Rocket is a mild amp usually not full of hum or buzz problems. I have one Rocket turned bass amp for my daughter with the cap board oriented like you first had your cap board with no problems.

If I am going to knock down some B+ voltage I use a reverse zener diode that is bolted to the chassis (heat sink) connected to CT of HT.

Mark

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